Merge pull request #3461 from bdarnell/deprecations-314

Python 3.14 deprecates the asyncio event loop policy system, so make (most of) the necessary changes.

The deprecation of set_event_loop is extremely disruptive to AsyncTestCase, so I've asked if it can remain undeprecated in python/cpython#130322. The testing.py changes are temporary until this is resolved.

Fixes #3458