Use async def instead of def ... -> Awaitable in typing

[sobolevn]

Source: https://github.com/python/cpython/blob/187930f74c44e460ba09c60ba5d9bb4fac543d8f/Lib/_collections_abc.py#L206-L240

Why is that important? Because async def is a Coroutine, which is much more than Awaitable. For example, this backfires in #6779

[sobolevn]

Ok, types.pyi as well.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

tornado (https://github.com/tornadoweb/tornado) + tornado/gen.py:429: error: Incompatible return value type (got "WaitIterator", expected "AsyncIterator[Any]")+ tornado/gen.py:429: note: Following member(s) of "WaitIterator" have conflicts:+ tornado/gen.py:429: note: Expected:+ tornado/gen.py:429: note: def __anext__(self) -> Coroutine[Any, Any, Any]+ tornado/gen.py:429: note: Got:+ tornado/gen.py:429: note: def __anext__(self) -> Future[Any]