ci: give the scripts a check for python3.9 compatibilty

[Ivan-Velickovic]

macOS ships Python 3.9 as the default version, we want the CI scripts to 'just work' which is why I want the CI scripts to work with Python 3.9.

Changing the CI scripts recently has made them fail for these older Python versions.

We should have some sort of check in CI (or get CI to just sue Python 3.9) to avoid subtle breaking changes.

[JDuchniewicz]

We seem to be using Python 3.12 in nix.flake - we should probably downgrade there to be compatible.