Python 3.14

[mirthebeijers]

Summary

Hi, this is my first contribution!!! Please be a little gentle with me ;-)

This is based on #3664 but I add fixes for

• in the test suite deprecations are marked as fatal, and uvicorn is used to set up the tests, and uvicorn generates a deprecation warning because of iscoroutinefunction()
• uvicorn is about to create a new release with a fix: Support Python 3.14 Kludex/uvicorn#2723 👀Python 3.14 has stricter requirements for event loops in background threads. The original code in httpx called server.run() which internally uses asyncio.run(), but this doesn't work properly in background threads in Python 3.14.
• asyncio.get_event_loop() deprecated: The TestServer.serve() method was using asyncio.get_event_loop() which is now more restrictive in Python 3.14.

I'm not fully sure if this also passes on 3.9, it might need a little more work, but on 3.14 I think this fixes the problems ;-)