Python: create infrastructure to test Python candidate releases

[isabelizimm]

Python 3.14 is currently planned to be released 2025-10-01. The current workflow has been to wait until all (or nearly all) dependencies are available in the latest Python version and then update our tests; we'd like to be a bit more proactive for future releases.

There's definitely a set of critical dependencies that must support the latest Python release; I imagine ipykernel and jedi are on that list. For others, we could use pytest's importorskip or skipif functionality to manage missing packages.