Revert "temporarily disable black" (#24)

This reverts commit b5e542f.

.github/workflows/actions.yml

@@ -18,6 +18,10 @@ jobs:
         uses: actions/checkout@v2
       - name: Setup Python
         uses: actions/setup-python@v2
+      - name: Lint (black)
+        uses: psf/black@stable
+        with:
+          args: ". --check"
       - name: Install dependencies
         run: |
           pip install -r requirements-test.txt