Give error if __exit__ returns False but is declared to return bool

[JukkaL]

Mypy can give false positives about missing return statements
if __exit__ that always returns False is annotated to return
bool instead of Literal[False].

Add new error code and documentation for the error code since
this error condition is not very obvious.

Fixes #7577.

There are two major limitations:

1. This doesn't support async context managers.
2. This won't help if a stub has an invalid __exit__ return type.

I'll create a follow-up issues about the above.

[Michael0x2a]

Maybe we could also suggest returning Optional[bool]? I believe that'll also make mypy assume the function will not swallow exceptions.

[msullivan]

Looks good. This will avoid a pretty tricky failure mode.

[JukkaL]

Maybe we could also suggest returning Optional[bool]? I believe that'll also make mypy assume the function will not swallow exceptions.

That doesn't sound correct, though. If the return type is Optional[bool], the function could still return True and swallow exceptions.