test: cap matplotlib<3.11 for the test env to stabilize image baselines

matplotlib 3.11.0 (released 2026-06-12) changes colorbar/image rasterization
enough to push tests/pl/test_colorbar.py image comparisons past the RMS
tolerance (RMS ~22-31 vs 15), failing CI repo-wide on the py3.11-stable env.

Cap matplotlib in the 'test' dependency-group only; the runtime dependency
stays open so users/downstream still get 3.11. Temporary until the colorbar
baselines are regenerated against 3.11.

Author: timtreis

pyproject.toml

@@ -48,6 +48,10 @@ dev = [
 ]
 test = [
   "coverage[toml]>=7.4",
+  # Image-comparison baselines are matplotlib-version-sensitive; matplotlib 3.11.0 (released
+  # 2026-06-12) shifts colorbar rendering past the comparison tolerance. Cap for tests only
+  # (the runtime dependency stays open) until the baselines are regenerated against 3.11.
+  "matplotlib<3.11",
   "pooch",
   "pytest",
   "pytest-cov",