GitHub Actions: Add Python 3.13 and 3.14

[cclauss]

Python v3.14 -- October 7th

• https://www.python.org/download/pre-releases
• https://www.python.org/downloads/release/python-3140rc3
• https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#allow-pre-releases
• https://pypy.org/download.html -- PyPy only keeps a single release as current.

Note: Python 3.8 was EOL one year ago. Python 3.9 will EOL in 10 days, when Python 3.14 will be released.

[pquentin]

Thanks! LGTM.

[pquentin]

We also need to fix the Sphinx configuration.

[Zac-HD]

Let's drop the 3.8 and 3.9 builds; I've marked them as not-required in branch protection rules.

[cclauss]

Thanks @Zac-HD ... Just one more Required py3.9 job to rename.

• Ubuntu (3.9, check docs) --> Ubuntu (3.12, check docs)

Do we need to solve the Sphinx config issue on this PR, or can we do that on a separate PR?

[Zac-HD]

let's do it on this pr!

[Zac-HD]

Thanks again, @cclauss!