chore: Add Django 5.1/5.2/6.0 to classifiers

[Michael-Deveau]

Extremely minor change, but this would be useful to get the PyPI page to pick up which Django versions are supported, similarly to how it does with the Python versions, which are already in the classifiers.
Explicit support in the classifiers would allow downstream usages to be informed about any Django version support changes as part of their CIs/automated processes, rather than having to parse the changelogs, etc. manually for that information. (And might prevent low-hanging questions about what the supported versions are.)

Django 5.1, 5.2, and 6.0 are the ones tested for in the pyproject+tox matrix as well as the GitHub Actions, so no actual changes are needed to commit to these as the supported versions, other than adding them to the classifiers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.33%. Comparing base (0588f48) to head (c5ad192).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #897   +/-   ##
=======================================
  Coverage   93.33%   93.33%           
=======================================
  Files          42       42           
  Lines        1350     1350           
  Branches       62       62           
=======================================
  Hits         1260     1260           
  Misses         67       67           
  Partials       23       23           

Flag	Coverage Δ
unittests	93.33% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[nijel]

Merged, thanks for your contribution!