Warn if __exit__ doesn't have the right signature.

[JelleZijlstra]

A typeshed user found that zipfile.ZipFile wasn't compatible with the ContextManager protocol, and it turns out that this was because its __exit__ method was declared as taking Exception instead of BaseException: python/typeshed#2043.

I imagine this kind of mistake is pretty common in user code, and it won't be noticed until somebody tries to pass the object to an API that accepts a ContextManager and the user gets an obscure error. Perhaps mypy should warn for __exit__ methods that are not compatible with the standard signature:

    def __exit__(self, t: Optional[Type[BaseException]] = ..., value: Optional[BaseException] = ...,
                 traceback: Optional[TracebackType] = ...) -> bool: ...

Doing this makes more sense for __exit__ than for many other special methods because __exit__'s signature is rather more complicated than that of other dunders, so it's easier to make mistakes. mypy already does something similar for binary operators.

[gvanrossum]

This sounds like a good idea. The signature of `__exit__` has indeed been the source of many tears.

[AlexWaygood]

We actually just implemented this in flake8-pyi (a flake8 plugin for linting stub files, which typeshed uses in its CI): PyCQA/flake8-pyi#199