Update requirements.txt and some CI config

[molpopgen]

• Update Ubuntu CI image
• Bump Python in CI to something not EOL
• rebuild requirements.txt using uv

[molpopgen]

@grahamgower

This has to happen before anything else can work (#167, etc.).

One proposal/question for this one:

I'd argue that we can take the requirements_old.txt and:

• rename it requirements.txt
• remove all the version pins
• rename the requirements.txt in this PR to requirements_locked.txt or requirements_CI.txt

A change like this would reproducible builds in CI, allow things like uv to regenerate the locked file for us, etc..

[grahamgower]

I like your suggested changes, @molpopgen! If using uv makes it easier to deal with the inevitable (and rapid) project bitrot, then I'm all for it!

[molpopgen]

I like your suggested changes, @molpopgen! If using uv makes it easier to deal with the inevitable (and rapid) project bitrot, then I'm all for it!

Sounds good -- I'll move those files around and update CI work flows accordingly. uv is surprisingly good! And even if Python keeps being Python and something else replaces it, it is still nice to not have to build these lock files by hand.

[molpopgen]

@grahamgower -- I made those changes and squashed it all down.

[grahamgower]

Merged. Thanks!