Add support for Python 3.9-3.14

[hugovk]

The Python 3.14 release candidate is out. 🚀

The 3.14 release manager (👋 hi, that's me!) said:

Call to action

We strongly encourage maintainers of third-party Python projects to prepare their projects for 3.14 during this phase, and publish Python 3.14 wheels on PyPI to be ready for the final release of 3.14.0, and to help other projects do their own testing. Any binary wheels built against Python 3.14.0rc1 will work with future versions of Python 3.14. As always, report any issues to the Python bug tracker.

This PR adds 3.13 and 3.14 to the CI, and the Trove classifiers for 3.9-3.14.

It also updates GitHub Actions.

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Separately, I recommend dropping support for EOL Python 3.6-3.8, but especially for 3.6-3.7 which are no longer tested on CI.