python.pkgs.pytest.setupHook: fix cache name, fixes #40273

The cache is not named `.pytest-cache` but `.pytest_cache`.

pkgs/development/python-modules/pytest/default.nix

···
     runHook postCheck
   '';
 
-  # Remove .pytest-cache when using py.test in a Nix build
+  # Remove .pytest_cache when using py.test in a Nix build
   setupHook = writeText "pytest-hook" ''
     postFixupHooks+=(
-        'find $out -name .pytest-cache -type d -exec rm -rf {} +'
+        'find $out -name .pytest_cache -type d -exec rm -rf {} +'
     )
   '';