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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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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