Update test matrix

[kevin-brown]

• [x] Remove Django 2.0
• [x] Remove Django 2.1
• [x] Add Django 3.2
• [x] Add Django 4.0
• [x] Add Python 3.9
• [x] Add Python 3.10

[kevin-brown]

Yeah the Python 3.8 tests fail for some unknown reason related to the version pinning within Black itself (the tests pass when run independently). And since Black is unpinned, newer versions bumped up the minimum requirements for Python 3.6 to be 3.6.2 to mitigate some bugs. So that was carried down to pytest-black to not affect the overall package requirements.

[stephrdev]

Yeah the Python 3.8 tests fail for some unknown reason related to the version pinning within Black itself (the tests pass when run independently). And since Black is unpinned, newer versions bumped up the minimum requirements for Python 3.6 to be 3.6.2 to mitigate some bugs. So that was carried down to pytest-black to not affect the overall package requirements.

I had a similar issue in another project and solved it by adding this dependency to the dev dependencies: https://github.com/stephrdev/django-cruditor/blob/main/pyproject.toml#L50

If you have time I'd appreciate a change on this PR to see if it works.