TYP: make IOHandles generic

[twoertwein]

Towards #41610

This doesn't help with any ignore statements, as Buffer is defined as Buffer = Union[IO[AnyStr], RawIOBase, BufferedIOBase, TextIOBase, TextIOWrapper, mmap]: IO[AnyStr] will be replaced with IO[str] or IO[bytes].

btw., Buffer is a "bad" union as it contains one generic and many actual types (I think, I'm the one to blame for that). It is often used without an actual type (Buffer instead of Buffer[str] or Buffer[bytes]).

[pep8speaks]

Hello @twoertwein! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-11-14 03:38:45 UTC

[twoertwein]

#43951 is a follow-up to this PR that also contains this PR. If #43951 isn't too large to review (it is large), I wouldn't mind simply skipping this PR.

[jreback]

lgtm @simonjayhawkins if any comments

[jreback]

can you merge merge master and can get this in

[twoertwein]

@jreback green'ish

[jreback]

thanks @twoertwein