Issue 24200.

Patch written by Ben Hoyt.

Remove "Availability: Unix, Windows." from os module documentation for
functions available on all platforms.

Provide __ne__ method for consistency.

Patch by Martin Panter, reviewed by Serhiy Storchaka and R. David Murray.

Test that asyncio.iscoroutine() supports 'async def' coroutines and
collections.abc.Coroutine types.