GitHub Actions: rework test matrix (drop Django 2.2, py3.9)

[asedeno]

• Drop Django 2.2 (EoL)
• Drop Python 3.9 build, since all supported Django versions support 3.10 now.

3.7  3.8  3.9  3.10
|----Django 3.2---|
     |-Django 4.0-|

[claudep]

Thanks but I'd rather do that after the next release, if possible. And for the next release, I'd like to merge #186. Do you have time for a quick review of that pull request?

[asedeno]

That's reasonable. I don't think this PR will age badly in the mean time. :-)