Add Python 3.13, drop EOL Python 3.8

[robhudson]

This also tweaks the tox config to take advantage of extras to install testing dependencies (which means we can use tox and tox-uv.

[janbrasna]

I'm wondering why there are still "required checks" pending for 3.8 blocking PR merging:

Could that be some of them are set in branch protection rules, so GH still waits for them?

• https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/collaborating-on-repositories-with-code-quality-features/troubleshooting-required-status-checks
• https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/about-protected-branches#require-status-checks-before-merging

They normally should turn off once they are not run for 7 days, these have been removed for more than a month… but still might be worth checking:

[robhudson]

I saw this from an email but couldn't find where it was after the fact. I fixed this in the settings. I appreciate you pointing this out.