Typing support for common proxy and delegation patterns

[y2kbugger (Zak)]

Adding a Proxy type to typing/typing_extensions

I spent the weekend thinking about this, and overall I think I like this idea! I just want to clarify a little bit about how AttrSpec would/should work: class MyProxy(Generic[A]): # is this right? def __getitem__(self, xyz: A.name) -> A.value: ... # or this def __getitem__(self, xyz: A.xyz) -> A.value: ... # what happens if you use A.name/A.value in non-dunder methods? # type error? Or do these just have to match the signature # of the method on A? def dtype(sel…