Add Python 3.15-dev to test matrix

[simonw]

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📚 Documentation preview 📚: https://datasette--2719.org.readthedocs.build/en/2719/

[simonw]

Here's where it failed:

  Building wheel for watchfiles (pyproject.toml): started
  Building wheel for watchfiles (pyproject.toml): finished with status 'error'
  error: subprocess-exited-with-error
  
  × Building wheel for watchfiles (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [67 lines of output]
      Running `maturin pep517 build-wheel -i 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (40e78e0) to head (e870ad2).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2719   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files         57      57           
  Lines       8776    8776           
=====================================
  Misses      8776    8776           

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