Remove continue-on-error from lint step

.github/workflows/tests.yml

@@ -25,7 +25,6 @@ jobs:
         python3 -m ensurepip --upgrade
         pip install --no-cache-dir -r requirements.txt -r requirements-dev.txt
     - name: Lint with pre-commit hook
-      continue-on-error: true # XXX temporary, until all code linted
       run: |
         pre-commit run --all-files
     - name: Test with pytest