Guido, Ben, Stefan, Nick,

I want to re-open this issue.  I'm still not satisfied with the current state of things, mainly with the __instancecheck__ hack for Awaitable & Coroutine ABCs (as Ben initially suggested).

I think we should remove the __instancecheck__ from the ABCs, and resurrect inspect.isawaitable() function:

1. abc.Coroutine and abc.Awaitable will guarantee that objects that implement them have '__await__' and it's safe to access it (that means that they will fail for generator-based coroutines).

2. inspect.isgenerator() can be used along with inspect.isawaitable() to detect generator-based coroutines (generators with CO_ITERABLE_COROUTINE flag).