Improve type hinting for ContextDecorator and AsyncContextDecorator

[ncoghlan]

I recently ran into the problem described in https://stackoverflow.com/questions/62703400/python-how-to-type-hint-a-callable-with-wrapped, where MyPy complained when I attempted to access __wrapped__ on a function decorated with a ContextDecorator subclass.

The function signature is handled correctly (as per #4399), but the fact ContextDecorator.__call__ necessarily adds the __wrapped__ attribute to the returned wrapper is lost.

The functools stub does have some machinery for accurately typing functools.wraps itself, but there isn't anything that could be readily used to adjust the ContextDecorator.__call__ signature.

Ideally something like the following would be possible:

    def __call__(self, func: _F) -> _WrappedCallable[_F]: ...

However, I'm not sure how _WrappedCallable could be expressed as a generic type - the corresponding protocol in functools.pyi accepts the call parameters and return type as separate type variables.