GitHub Actions: fast-fail on black formatting issues

Give fast feedback to contributors https://github.com/psf/black#github-actions

Author: cclauss

.github/workflows/pre-commit.yml

@@ -14,6 +14,7 @@ jobs:
             ~/.cache/pip
           key: ${{ runner.os }}-pre-commit-${{ hashFiles('.pre-commit-config.yaml') }}
       - uses: actions/setup-python@v2
+      - uses: psf/black@stable
       - name: Install pre-commit
         run: |
           python -m pip install --upgrade pip