Drop unsupported versions of python and Django

[mikemanger]

Probably doesn't warrant a new release but I think it makes sense to drop support for unsupported versions of Django from the test matrix.

• Test against Django 4.2 - 5.1 (4.2 was missing before)
• Drop support for EOL Python 3.8
• Use Python 3.9 generic types

[sklarsa]

Clearly I've been out of the Django game for a while :-) Thanks again for all of the work on this repo!