Update tests and drop EOL versions of Python and Django

[mikemanger]

This PR has no user facing changes, it continues the work done in #147

• Drop EOL versions of (Django < 4.2) and Python 3.8
• Tests against Django 5.2, Python 3.12/3.13 and PyPy 3.11
• Simplified test matrix to just use tox.ini for Django versions (this knocks off about 2 minutes from the run time)
• Updates GitHub actions to fix warnings
• Adds pre-commit repos for django-upgrade, pyupgrade and standard pre-commit-hooks
• Uses python 3 f strings in some tests
• Fixes some whitespace issues
• Also updates some link in the readme

Note I've bumped the release workflow actions - as far as I can tell they should still work fine with no config changes but I've not verified this.