Subprocess crashes in `test_download_parallel_from_internet_works`

### Description
On Python 3.6-3.7 (at least) one of the processes spawned in the above test segfaults.

### Actual behavior
```
../../sw/lib/python3.7/site-packages/astropy/utils/tests/test_data.py::test_download_parallel_from_internet_works Fatal Python error: Segmentation fault

Current thread 0x00007fffa857e3c0 (most recent call first):
  File "/sw/lib/python3.7/socket.py", line 748 in getaddrinfo
  File "/sw/lib/python3.7/socket.py", line 707 in create_connection
  File "/sw/lib/python3.7/http/client.py", line 938 in connect
  File "/sw/lib/python3.7/http/client.py", line 966 in send
  File "/sw/lib/python3.7/http/client.py", line 1026 in _send_output
  File "/sw/lib/python3.7/http/client.py", line 1247 in endheaders
  File "/sw/lib/python3.7/http/client.py", line 1298 in _send_request
  File "/sw/lib/python3.7/http/client.py", line 1252 in request
  File "/sw/lib/python3.7/urllib/request.py", line 1317 in do_open
  File "/sw/lib/python3.7/urllib/request.py", line 1345 in http_open
  File "/sw/lib/python3.7/urllib/request.py", line 503 in _call_chain
  File "/sw/lib/python3.7/urllib/request.py", line 543 in _open
  File "/sw/lib/python3.7/urllib/request.py", line 525 in open
  File "/sw/lib/python3.7/urllib/request.py", line 222 in urlopen
  File "/sw/lib/python3.7/site-packages/astropy/utils/data.py", line 979 in _download_file_from_source
  File "/sw/lib/python3.7/site-packages/astropy/utils/data.py", line 1147 in download_file
  File "/sw/lib/python3.7/site-packages/astropy/utils/data.py", line 1243 in _do_download_files_in_parallel
  File "/sw/lib/python3.7/site-packages/astropy/utils/console.py", line 499 in __call__
  File "/sw/lib/python3.7/concurrent/futures/process.py", line 239 in _process_worker
  File "/sw/lib/python3.7/multiprocessing/process.py", line 99 in run
  File "/sw/lib/python3.7/multiprocessing/process.py", line 297 in _bootstrap
  File "/sw/lib/python3.7/multiprocessing/popen_fork.py", line 74 in _launch
  File "/sw/lib/python3.7/multiprocessing/popen_fork.py", line 20 in __init__
  File "/sw/lib/python3.7/multiprocessing/context.py", line 277 in _Popen
  File "/sw/lib/python3.7/multiprocessing/process.py", line 112 in start
  File "/sw/lib/python3.7/concurrent/futures/process.py", line 607 in _adjust_process_count
  File "/sw/lib/python3.7/concurrent/futures/process.py", line 583 in _start_queue_management_thread
  File "/sw/lib/python3.7/concurrent/futures/process.py", line 641 in submit
  File "/sw/lib/python3.7/site-packages/astropy/utils/console.py", line 850 in <genexpr>
  File "/sw/lib/python3.7/concurrent/futures/_base.py", line 217 in as_completed
  File "/sw/lib/python3.7/site-packages/astropy/utils/console.py", line 850 in map_unordered
  File "/sw/lib/python3.7/site-packages/astropy/utils/console.py", line 756 in map
  File "/sw/lib/python3.7/site-packages/astropy/utils/data.py", line 1362 in download_files_in_parallel
  File "/sw/lib/python3.7/site-packages/astropy/utils/tests/test_data.py", line 525 in test_download_parallel_from_internet_works
  File "/sw/lib/python3.7/site-packages/_pytest/python.py", line 176 in pytest_pyfunc_call
  File "/sw/lib/python3.7/site-packages/pluggy/callers.py", line 187 in _multicall
  File "/sw/lib/python3.7/site-packages/pluggy/manager.py", line 86 in <lambda>
  File "/sw/lib/python3.7/site-packages/pluggy/manager.py", line 92 in _hookexec
  File "/sw/lib/python3.7/site-packages/pluggy/hooks.py", line 286 in __call__
  File "/sw/lib/python3.7/site-packages/_pytest/python.py", line 1445 in runtest
  File "/sw/lib/python3.7/site-packages/_pytest/runner.py", line 126 in pytest_runtest_call
  File "/sw/lib/python3.7/site-packages/pluggy/callers.py", line 187 in _multicall
  File "/sw/lib/python3.7/site-packages/pluggy/manager.py", line 86 in <lambda>
  File "/sw/lib/python3.7/site-packages/pluggy/manager.py", line 92 in _hookexec
  File "/sw/lib/python3.7/site-packages/pluggy/hooks.py", line 286 in __call__
  File "/sw/lib/python3.7/site-packages/flaky/flaky_pytest_plugin.py", line 306 in <lambda>
  File "/sw/lib/python3.7/site-packages/_pytest/runner.py", line 229 in from_call
  File "/sw/lib/python3.7/site-packages/flaky/flaky_pytest_plugin.py", line 307 in call_runtest_hook
  File "/sw/lib/python3.7/site-packages/flaky/flaky_pytest_plugin.py", line 129 in call_and_report
  File "/sw/lib/python3.7/site-packages/_pytest/runner.py", line 96 in runtestprotocol
  File "/sw/lib/python3.7/site-packages/_pytest/runner.py", line 81 in pytest_runtest_protocol
  File "/sw/lib/python3.7/site-packages/flaky/flaky_pytest_plugin.py", line 92 in pytest_runtest_protocol
  File "/sw/lib/python3.7/site-packages/pluggy/callers.py", line 187 in _multicall
  File "/sw/lib/python3.7/site-packages/pluggy/manager.py", line 86 in <lambda>
  File "/sw/lib/python3.7/site-packages/pluggy/manager.py", line 92 in _hookexec
  File "/sw/lib/python3.7/site-packages/pluggy/hooks.py", line 286 in __call__
  File "/sw/lib/python3.7/site-packages/_pytest/main.py", line 264 in pytest_runtestloop
  File "/sw/lib/python3.7/site-packages/pluggy/callers.py", line 187 in _multicall
  File "/sw/lib/python3.7/site-packages/pluggy/manager.py", line 86 in <lambda>
  File "/sw/lib/python3.7/site-packages/pluggy/manager.py", line 92 in _hookexec
  File "/sw/lib/python3.7/site-packages/pluggy/hooks.py", line 286 in __call__
  File "/sw/lib/python3.7/site-packages/_pytest/main.py", line 240 in _main
  File "/sw/lib/python3.7/site-packages/_pytest/main.py", line 196 in wrap_session
  File "/sw/lib/python3.7/site-packages/_pytest/main.py", line 233 in pytest_cmdline_main
  File "/sw/lib/python3.7/site-packages/pluggy/callers.py", line 187 in _multicall
  File "/sw/lib/python3.7/site-packages/pluggy/manager.py", line 86 in <lambda>
  File "/sw/lib/python3.7/site-packages/pluggy/manager.py", line 92 in _hookexec
  File "/sw/lib/python3.7/site-packages/pluggy/hooks.py", line 286 in __call__
  File "/sw/lib/python3.7/site-packages/_pytest/config/__init__.py", line 92 in main
  File "/sw/lib/python3.7/site-packages/astropy/tests/runner.py", line 248 in run_tests
  File "/sw/lib/python3.7/site-packages/astropy/tests/runner.py", line 611 in run_tests
  File "/sw/lib/python3.7/site-packages/astropy/tests/runner.py", line 266 in test
  File "<string>", line 1 in <module>
FAILED                      [ 34%]
```
```
____________________________________________________ test_download_parallel_from_internet_works _____________________________________________________

temp_cache = None

    @pytest.mark.remote_data(source="astropy")
    def test_download_parallel_from_internet_works(temp_cache):
        main_url = conf.dataurl
        mirror_url = conf.dataurl_mirror
        fileloc = "intersphinx/README"
        urls = []
        sources = {}
        for s in ["", fileloc]:
            urls.append(main_url + s)
            sources[urls[-1]] = [urls[-1], mirror_url+s]
>       fnout = download_files_in_parallel(urls, sources=sources)

/sw/lib/python3.7/site-packages/astropy/utils/tests/test_data.py:525: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/sw/lib/python3.7/site-packages/astropy/utils/data.py:1362: in download_files_in_parallel
    multiprocessing_start_method=multiprocessing_start_method,
/sw/lib/python3.7/site-packages/astropy/utils/console.py:756: in map
    multiprocessing_start_method=multiprocessing_start_method)
/sw/lib/python3.7/site-packages/astropy/utils/console.py:852: in map_unordered
    results.append(f.result())
/sw/lib/python3.7/concurrent/futures/_base.py:428: in result
    return self.__get_result()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <Future at 0x1184adb90 state=finished raised BrokenProcessPool>

    def __get_result(self):
        if self._exception:
>           raise self._exception
E           concurrent.futures.process.BrokenProcessPool: A process in the process pool was terminated abruptly while the future was running or pending.

/sw/lib/python3.7/concurrent/futures/_base.py:384: BrokenProcessPool
```
### Steps to Reproduce
Apparently this only happens when (presumably) `test_open_network_url` from the suite of `utils` tests is previously run, i.e. 
```
python -c 'import astropy; astropy.test(package="utils", args="-vk \" _open_network_url or _download\"", remote_data=True)'
```
does trigger the error, while
```
python -c 'import astropy; astropy.test(package="utils", args="-vk _download", remote_data=True)'
```
on its own does not.
Arguably this has improved compared to 3.2.3, where the main Python process would become locked up as well.
Everything succeeds on Python 3.8.0

### System Details
```
platform darwin -- Python 3.7.5, pytest-5.3.0, py-1.8.0, pluggy-0.13.0 -- /sw/bin/python3.7
cachedir: .pytest_cache
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('/Users/derek/.hypothesis/examples')

Running tests with Astropy version 4.0rc1.
Running tests in astropy/utils.

Date: 2019-11-27T21:11:09

Platform: Darwin-16.7.0-x86_64-i386-64bit

Executable: /sw/bin/python3.7

Full Python Version: 
3.7.5 (default, Nov  2 2019, 21:26:51) 
[Clang 9.0.0 (clang-900.0.39.2)]

encodings: sys: utf-8, locale: UTF-8, filesystem: utf-8
byteorder: little
float info: dig: 15, mant_dig: 15

Package versions: 
Numpy: 1.17.4
Scipy: 1.3.2
Matplotlib: 3.1.0
h5py: 2.9.0
Pandas: 0.25.3
Cython: 0.29.13
Scikit-image: not available
asdf: 2.4.2
astropy-helpers: 4.0.rc1

Using Astropy options: remote_data: any.

Matplotlib: 3.1.0
Freetype: 2.6.1
rootdir: /sw/lib/python3.7/site-packages
plugins: asdf-2.4.2, flaky-3.5.3, hypothesis-4.36.0, arraydiff-0.3, astropy-header-0.1.1, cov-2.6.0, doctestplus-0.5.0, flake8-1.0.4, forked-1.1.3, mock-1.10.4, mpl-0.11, openfiles-0.3.2, remotedata-0.3.1
collected 301 items / 240 deselected / 61 selected
```

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Subprocess crashes in `test_download_parallel_from_internet_works` #9699

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Uh oh!

Subprocess crashes in test_download_parallel_from_internet_works #9699

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Subprocess crashes in `test_download_parallel_from_internet_works` #9699