Use exact versions in Python matrix

[puddly]

I'm noticing a lot of CI failures today that seem to stem from GitHub actions not using a consistent minor version:

# Set up Python 3.12
Run actions/setup-python@v5
Installed versions
  Successfully set up CPython (3.12.9)
                               ^^^^^^
...
# Restore base Python virtual environment
Error: Failed to restore cache entry. Exiting as fail-on-cache-miss is set.
Input key: 2-Linux-base-venv-3.12.10-52ed2b2b97441a5669110477fc580f538eeeab0617890c647a04f116296c1809
                             ^^^^^^^

[TheJulianJES]

hmm, that's interesting