Fix tox ci

[tbrlpld]

This PR fixes the tox configuration in CI.

The gh-actions:env section was not needed. It actually led to not all environment combinations being run and it also meant that the dependency installation did not happen in the expected environments.

See: ymyzk/tox-gh-actions#193

One symptom of the above was the we never actually tested with Django 3.2 in CI (which was noticed because it led to errors when running tox locally, where the environments were working just fine). Now we are properly testing Django 3.2 on CI as well.

Another symptom of the above was that we needed to redefine coverage in the tox.ini dependencies although it is already listed in the testing dependencies in pyproject.toml.

The redefinition of coverage was also solved (before the above complete solution to the environment issue) previously by switching from skipsdist, usedevelop and a custom install_command to the more modern tox config options extras = testing and package = editable.

Also bumps CI versions to not use deprecated Node version 16 anymore.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fe8a7ec) 99.52% compared to head (41fa424) 100.00%.
Report is 20 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #10      +/-   ##
===========================================
+ Coverage   99.52%   100.00%   +0.47%     
===========================================
  Files          12        12              
  Lines         212       212              
===========================================
+ Hits          211       212       +1     
+ Misses          1         0       -1     

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