os.linesep can only be \n or \r\n

[sobolevn]

See:

• https://github.com/python/cpython/blob/5334732f9c8a44722e4b339f4bb837b5b0226991/Lib/os.py#L52-L54 for POSIX
• https://github.com/python/cpython/blob/5334732f9c8a44722e4b339f4bb837b5b0226991/Lib/os.py#L76-L78 on Windows

[srittau]

The primer hit – while only in test cases – in my opinion shows why it's better not to distinguish this by platform: Code should be able to handle both \r\n and \n, but type checkers would not enforce that – unless you run the type checkers on multiple platforms.

[github-actions]

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