GitHub Actions: rework test matrix

[asedeno]

Drop Django 3.1 (EoL)
Add Django 4.0
Add Python 3.8 (Minimum version for Django 4.0)
Add Python 3.10 to the general matrix, not just for Djagno 3.2

Exclude Django 2.2 from Python 3.10 testing.
Exclude Django 4.0 from Python 3.7 testing.

Django 2.2 will EoL in a little over a month, at which point we can
trim the matrix down again by dropping it and Python 3.9 if we want
to.

[coveralls]

Coverage remained the same at 97.492% when pulling 4432985 on asedeno:django4 into 65adf90 on notanumber:master.

[claudep]

By the way, if you find a way to reduce the coveralls runs to only one, it would be sufficient and nice.