Change IO/IOBase.__exit__ return type to None

[nbdaaron]

similar to #7470, this was caught by quora/pyanalyze#496.

I don't believe __exit__ in IOBase returns anything: https://github.com/python/cpython/blob/main/Lib/_pyio.py#L513 and I think typing.IO just represents I/O streams returned by open() which I don't think return anything from __exit__

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

zulip (https://github.com/zulip/zulip)
+ zerver/lib/test_console_output.py:123: error: Return type "Optional[bool]" of "__exit__" incompatible with return type "None" in supertype "IO"  [override]

[JelleZijlstra]

The C version of IOBase.__exit__ just returns self.close(), and close() is expected to return None.

[srittau]

While I think it's okay to change the return type for IOBase if the subclasses all return None, I'm not sure we should do the same for IO as it is a semi-protocol and sub-classes could decide to return True.

[JelleZijlstra]

I feel like the contract for IO.__exit__ is that it closes the file. A subclass that also suppresses exceptions would be unexpected enough that it's good for a type checker to warn about it.