UnicodeDecodeError on Windows with pyproject.toml

**Crash Report**

On Windows (where the default encoding for `open` is usually not UTF-8), with a `pyproject.toml` file containing the mypy configuration, mypy crashes due to being unable to decode unicode characters.

The [TOML specification](https://toml.io/en/v1.0.0#spec) requires files to be encoded in UTF-8, so it is valid to assume tools will correctly parse files containing unicode characters.

**Traceback**

```python
Traceback (most recent call last):
  File "C:\hostedtoolcache\windows\Python\3.6.8\x64\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\hostedtoolcache\windows\Python\3.6.8\x64\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "D:\a\domdf_python_tools\domdf_python_tools\.tox\mypy\Scripts\mypy.EXE\__main__.py", line 7, in <module>
  File "D:\a\domdf_python_tools\domdf_python_tools\.tox\mypy\lib\site-packages\mypy\__main__.py", line 11, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "mypy\main.py", line 87, in main
  File "mypy\main.py", line 165, in run_build
  File "mypy\build.py", line 179, in build
  File "mypy\build.py", line 229, in _build
  File "mypy\build.py", line 475, in load_plugins
  File "mypy\build.py", line 391, in load_plugins_from_config
  File "mypy\build.py", line 511, in find_config_file_line_number
  File "C:\hostedtoolcache\windows\Python\3.6.8\x64\lib\encodings\cp1252.py", line 23, in decode
    return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x90 in position 220: character maps to <undefined>
```

This is caused by the following function, particularly the call to `open` on line 511 without an encoding being specified.

https://github.com/python/mypy/blob/5c4aea39ab6a14eeef85cc849d6057bebf2147a3/mypy/build.py#L503-L523

**To Reproduce**

* Use the following `pyproject.toml` file content:
```toml
[project]
description = "Factory ⸻ A code generator 🏭"

[tool.mypy]
```

* Run mypy 0.910 on Windows

**Your Environment**



- Mypy version used: 0.910
- Mypy command-line flags: None
- Mypy configuration options from `mypy.ini` (and other config files): None (empty `tool.mypy` section in `pyproject.toml`)
- Python version used: 3.6 and 3.9
- Operating system and version: Windows 10

	def find_config_file_line_number(path: str, section: str, setting_name: str) -> int:
	"""Return the approximate location of setting_name within mypy config file.

	Return -1 if can't determine the line unambiguously.
	"""
	in_desired_section = False
	try:
	results = []
	with open(path) as f:
	for i, line in enumerate(f):
	line = line.strip()
	if line.startswith('[') and line.endswith(']'):
	current_section = line[1:-1].strip()
	in_desired_section = (current_section == section)
	elif in_desired_section and re.match(r'{}\s*='.format(setting_name), line):
	results.append(i + 1)
	if len(results) == 1:
	return results[0]
	except OSError:
	pass
	return -1

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UnicodeDecodeError on Windows with pyproject.toml #11058

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