GitHub Actions: rework test matrix #222

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.