This causes test_ntpath and test_posixpath to fail in subj.

Sample failure:

======================================================================
FAIL: test_expandvars_nonascii (__main__.NtCommonTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\Users\Sasha\Documents\cpython\lib\test\test_genericpath.py", line 230
, in test_expandvars_nonascii
    check(u'$spam bar', u'%s bar' % unonascii)
  File "c:\Users\Sasha\Documents\cpython\lib\test\test_genericpath.py", line 214
, in check
    self.assertEqual(expandvars(value), expected)
AssertionError: u'? bar' != u'\xe6 bar'
- ? bar
? ^
+ \xe6 bar
? ^


Cause:

if sys.getfilesystemencoding()=='mbcs', encoding Unicode characters that are missing in the current locale succeeds but produces '?'.

So, test.support.FS_NONASCII's test fails to detect if a character is present in the current locale.

> FAIL: test_expandvars_nonascii (__main__.NtCommonTest)

What is your Python version?

Python uses UTF-8 to encode paths on Windows since Python 3.6:
https://vstinner.github.io/python36-utf8-windows.html

In 3.x, it turns out, this doesn't result in test failures in stock configuration.

It does though if PYTHONLEGACYWINDOWSFSENCODING is in system environment.

I was diagnosing failures in 2.x and saw that 3.x has the same logic, so it was a no-brainer to replicate the change...

New changeset 8752dfbd1f0c96ca09cdacabaf0d0f8c3895b6ce by Serhiy Storchaka (native-api) in branch 'master':
bpo-33709: test_ntpath and test_posixpath fail in Windows with ACP!=1252. (GH-7278)
https://github.com/python/cpython/commit/8752dfbd1f0c96ca09cdacabaf0d0f8c3895b6ce

New changeset b1438c0d376e1d438a11927e2698e3317da0d854 by Miss Islington (bot) in branch '3.7':
bpo-33709: test_ntpath and test_posixpath fail in Windows with ACP!=1252. (GH-7278)
https://github.com/python/cpython/commit/b1438c0d376e1d438a11927e2698e3317da0d854

New changeset af31228650d30f02a283d291ba106e84275a04c1 by Miss Islington (bot) in branch '3.6':
bpo-33709: test_ntpath and test_posixpath fail in Windows with ACP!=1252. (GH-7278)
https://github.com/python/cpython/commit/af31228650d30f02a283d291ba106e84275a04c1

New changeset 29a4cbff92862207eb9df9a970b3636b8b06ff5d by Serhiy Storchaka (native-api) in branch '2.7':
[2.7] bpo-33709: test_ntpath and test_posixpath fail in Windows with ACP!=1252. (GH-7278) (GH-7279)
https://github.com/python/cpython/commit/29a4cbff92862207eb9df9a970b3636b8b06ff5d