Typing support for common proxy and delegation patterns

[blhsing (Ben Hsing)]

@srittau

Also, this would be most useful if it would work with generics, since proxies can often work with multiple proxy targets.

@jesnie

from typing import Proxy

class MyProxy(Proxy[Subject]):
    ...

class MyGenericProxy[S](Proxy[S]):

Ah yes, support for generics is crucial indeed, and I do like the proposed magic superclass.