Add Django 6.1 support

[codingjoe]

Support Django 5.2 through 6.1.

• Add the Django 6.1 classifier
• Test Django 5.2, 6.0 and 6.1 in CI

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.27%. Comparing base (fb134cd) to head (735cb97).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #381   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files           7        7           
  Lines         277      277           
=======================================
  Hits          275      275           
  Misses          2        2           

Flag	Coverage Δ
python-3.14	98.19% <ø> (ø)
selenium-ubuntu-latest	83.39% <ø> (ø)

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