why: simply motivated by Python's own EOL-cycle; i.e. I'm not
actively going to use any 3.13 or 3.9 features now.
how: updating test-matrixes; but no actual local testing done yet
I briefly considered making this a separate job, but I can't really justify that;
the 'separate' job would have to do all kinds of similar setup (pip install), and
to what end really? Broken is broken, I just want a notification. I'll see quickly
enought that it's about makemigrations