bpo-32528: Document the change in inheritance of asyncio.CancelledError (GH-21474)

#msg373510

[bpo-32528]()/#13528 changed `asyncio.CancelledError` such that it no longer inherits from `concurrent.futures.CancelledError`. As this affects existing code, specifically when catching the latter instead of the former in exception handling, it should be documented in the "What's new in 3.8?" document.

Automerge-Triggered-By: @1st1