CI runs intended for Python 3.9 actually use Python 3.12 instead

[maresb]

Oops, this one's pretty embarrassing.

We'll need separate lockfiles for each Python version we test.

The strategy I have in mind is to create separate environment-python-3.X.yml files for each Python version, and then run the lock command on both (simultaneously) the primary environment.yml and environment-python-3.X.yml.

That description was probably confusing, so it's probably easier to look at the implementation I did in pymc-labs/CausalPy#281.

With this scheme I find it slightly awkward to maintain separate files just to specify the desired Python version, but ultimately I think it should be very little effort. (I'm open to alternative suggestions.)

RELATED: If we want the upcoming release to support micromamba v1, we should add that to the matrix as well.

[maresb]

Ping @romain-intel since you expressed interest in helping out 🙏