Pin pytest to work around API changes

[AlexanderWells-diamond]

I can't see exactly what it is, but it must be somewhere in here: https://docs.pytest.org/en/stable/changelog.html#pytest-9-1-0-2026-06-13

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.39%. Comparing base (2981d7d) to head (c47ad75).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #207   +/-   ##
=======================================
  Coverage   88.39%   88.39%           
=======================================
  Files          15       15           
  Lines        1258     1258           
=======================================
  Hits         1112     1112           
  Misses        146      146           

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