Run ruff check in CI again

It had been disabled by d9756f0.

Remove `--diff` as it actually silences issues.

.github/workflows/pythonpackage.yml

@@ -81,7 +81,8 @@ jobs:
     - uses: actions/checkout@v4
     - name: Set up Python
       uses: actions/setup-python@v5
-    - run: pipx run ruff format --diff
+    - run: pipx run ruff check
+    - run: pipx run ruff format
 
   codespell:
     runs-on: ubuntu-latest