Remove typing.AwaitableGenerator

[hauntsaninja]

As mentioned in the code, AwaitableGenerator doesn't exist in typing.py or PEP 484.

typeshed/stdlib/3/typing.pyi

Line 208 in 626a0f3

# NOTE: This type does not exist in typing.py or PEP 484.

It appears it's an "intentionally undocumented" implementation detail of mypy, as mentioned in python/mypy#2907. Ideally, these shouldn't leak into typeshed. I've created issue python/mypy#8699 in mypy, since that's where action needs to be taken first, but keeping an issue here for tracking.

[hauntsaninja]

Similar to #3556 and #2999

[AlexWaygood]

I'm closing this as we now have #7580, and I don't think we need both issues to be open when they're about the same thing.