Specify file encoding

[Avasam]

See https://peps.python.org/pep-0597/
This also happened to me on Windows in #8879 because of mathematical unicode characters as part of the comments:

(.venv) PS C:\Users\Avasam\Documents\Git\typeshed> python .\tests\check_new_syntax.py
Traceback (most recent call last):
  File "C:\Users\Avasam\Documents\Git\typeshed\tests\check_new_syntax.py", line 110, in <module>
    main()
  File "C:\Users\Avasam\Documents\Git\typeshed\tests\check_new_syntax.py", line 100, in main
    stub = f.read()
  File "C:\Program Files\Python39\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 176166: 
character maps to <undefined>

There are linters to catch this, namely pylint (unspecified-encoding), flake8-encodings and flake8-file-encoding. I have tried both flake8 plugins mentioned and they gave me the same results, other than slightly different error messages:

[JelleZijlstra]

Thanks! This is generally an issue on Windows only since Unix defaults to utf-8.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[AlexWaygood]

Thanks!

[AlexWaygood]

The code in this file is never actually run (it's a test-case), but I guess there's no way of telling the flake8 plugin that, and I don't think this change does any actual harm :)