Pin pytest to work around API changes

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

Author: AlexanderWells-diamond

setup.cfg

@@ -46,6 +46,8 @@ useful =
     aioca >=1.6
 # Dev and docs dependencies
 dev =
+    # Pytest 9.1.0 deprecates some APIs that some of our plugins require
+    pytest<9.1.0
     pytest-cov
     pytest-flake8
     # Flake8 5.0 starts printing error, so pin to avoid it: