Add typing.Generator

[JukkaL]

typing.Generator is specified in the PEP 484 draft. Allow it to be used as the return type of a generator function.

[spkersten]

I'm trying to implement this (see my generator branch for progress). Should Iterator still be allowed as return type of a function in case Generator's send_type and return_type are None?

[JukkaL]

If a function has Generator as the return type, it should not return an Iterator, as Iterator may not have the send and throw methods.

[gvanrossum]

See also #695 and #1011 (at least one of these three is a duplicate).

FWIW I think Sander's question is whether this is still allowed:

def foo() -> Iterator[int]:
    yield 0

or whether all generators must be declared as Generator. I believe this example should still be valid (but callers should not call send() or throw().)

[JukkaL]

Yeah, an Iterator return type should still be fine. I was confused above about the question.

[gvanrossum]

Also closed by #1128.