Update supported versions of django

[mfogel]

• drop support for unsupported versions of django
• officially add support for django 5.1, even though it's not stable yet

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.09%. Comparing base (e1c8879) to head (f0c3205).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #136 +/- ## ======================================= Coverage 99.09% 99.09% ======================================= Files 10 10 Lines 222 222 ======================================= Hits 220 220 Misses 2 2 ```

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