Increase django version and switch to pyproject.toml

[mikemanger]

Includes changes in #39. I thought I'd split out the pre-commit and doc fixes in-case this PR goes too far. You can view the diff here

• Increases Django requirements to 4.2 and Python to 3.8 to fix #40
• Test against Django 5.1
• Moves setup.py and dev tool config to pyproject.toml
• Adds django-upgrade pre-commit task
• Fixes link in readme

[mpasternak]

Hey @mikemanger thank you very much for your kind contribution, I will review those changes and merge some time later this week; thank you very much.