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

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

GH-msg373510

[bpo-32528]()/GH-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
(cherry picked from commit 2a5181829af394b82e8e8c917183c709ee72a2b7)

Co-authored-by: JustAnotherArchivist <JustAnotherArchivist@users.noreply.github.com>