Drop Python 3.8 and Django 5.0, add Python 3.13 and Django 5.10, update old code and fix tests

[mikemanger]

This PR:

• Drops testing for Django 5.0 (5.0 is EOL in April 2025)
• Drops support for Python 3.8
• Tests against Django 5.1, 5.2 alpha and Python 3.13
• Fixes tests for the dev branch of Django 6.0
• Updates codebase to use Python 3 syntax for objects, strings and super calls
• Uses Django 4.2 syntax for setting RequestFactory headers
• Fixes warnings in GitHub actions
• Adds pre-commit config for django-upgrade and pyupgrade

[ddabble]

I'm not a huge fan of dropping support for Django 5.0 before it has actually reached EOL, but since I think it's very unlikely that any changes will be made to that version that we will have to accommodate in the next 2 months, it's fine 👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (17fc0b6) to head (79e442b).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #171   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           11        11           
  Lines          400       400           
  Branches        68        54   -14     
=========================================
  Hits           400       400           

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[ddabble]

Fantastic! Really appreciate that you took the initiative to make these changes, including adding the two very useful pre-commit hooks 😄