Improve testing story for Python 3.14 and free-threading builds

[Zac-HD]

#4025 got some initial wins - like "running 3.14 in CI" and "ensuring that our auto-updates set up free-threading environments for us", but there's plenty more to do

• turn on CI for the free-threading builds (e.g. 3.13.0t-dev) and see what breaks. Might be blocked on upstream pydata packages for extensions? We also don't guarantee thread-safety at the moment, so I expect some chaos here and we might just turn the tests off again for a while.
• search for FIXME-py314 and... fix them
  • with the removal of typing.Bytestring, we no longer generate bytes instances from Sequence[int]; I think we probably should still do this
  • test_suggests_elements_instead_of_annotations is failing, so it seems something about dataclass introspection changed
• fix any other 3.14-specific issues that come up if or when they arise