Various updates

[blag]

• Adjust the support matrix to drop Django < 3.2 and add Django > 4.
• Update action versions in GHA config
• Drop flake8 and isort, replace with ruff
• Convert from setup.py to setup.cfg and then to only pyproject.toml
• Add pre-commit checks
• Drop compat with really old Django versions
• Update tests/settings.py a bit
• Fix tests for Django 4.1+

The only differences that are user-facing are the changes in supported versions. But I think it's reasonable to reduce the support matrix down to only what Django officially still supports, and what is supported by GHA.