Python 3.14 support

[alexmojaki]

No pressure, but since Python 3.14 is released I just want to be able to follow progress on this.

[aabmass]

We should probably test against the free threading build as well

[jenshnielsen]

The only issue I have noticed is a DeprecationWarning triggered for asyncio.iscoroutinefunction python/cpython#122875

[alexmojaki]

That seems fixed in #4786, so maybe just adding 3.14 to tox and GHA is enough?