Bump supported cpython version to 3.14 for testing

[clin1234]

This is to mainly silence this abominable error:

        error: the configured Python interpreter version (3.14) is newer than PyO3's maximum supported version (3.13)
        = help: please check if an updated version of PyO3 is available. Current version: 0.23.1

[davidhewitt]

Thanks for the PR. I am not comfortable bumping this check at this point. If we did, unsuspecting users will likely install software built with the next PyO3 version (call it 0.24) when 3.14 stable lands... and they will likely have a terrible experience with random crashes and other frustrating hard-to-debug and dangerous issues.

At the same time, I appreciate the desire to get testing with 3.14 and that PyO3 should not be a blocker for this. See #4662 (comment)
for my latest suggestion on what to do here.

We also have an undocumented environment variable UNSAFE_PYO3_SKIP_VERSION_CHECK=1 which can be used for development / testing. We could document that option, though with the heavy caveat that it's highly likely any unsupported version of Python will crash pretty quickly.

If you are testing PyO3 with 3.14 you may need to make patches to pyo3-ffi to match any changes in the current alphas. I think I'm ok with having such patches in PyO3 to allow testing to make progress, though of course we will not guarantee any stability until at least the beta versions land.