Update tested versions

[adamchainz]

Made these version changes:

1. Drop EOL Python versions 3.7 and 3.8.
2. Add Python 3.13.
3. Drop Django 3.2, 4.0, and 4.1.
4. Add Django up to 5.2 (currently in beta, expected April).

On the way, I made these extra testing changes:

1. Fixed tox to run tests.py directly. The setup.py test command was removed in setuptools 72.0.0. This is a fine replacement for now.
2. Made tox run per-Django-version tests.
3. Made GitHub Actions run all configured Django versions’ tests per Python version. This is a pattern I use in my own projects (example) that allows tests to run in parallel without repeating the matrix in GitHub Actions. For small test suites, it ends up being faster than one job per Python + Django version, because of caching.