adjust ci settings to match pretalx settings

.github/workflows/style.yml

@@ -61,6 +61,30 @@ jobs:
       - name: Run flake8
         run: flake8 .
         working-directory: .
+  black:
+    name: black
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: install gettext
+        run: sudo apt install gettext
+      - name: Set up Python 3.8
+        uses: actions/setup-python@v1
+        with:
+          python-version: 3.8
+      - uses: actions/cache@v1
+        with:
+          path: ~/.cache/pip
+          key: ${{ runner.os }}-pip-${{ hashFiles('**/setup.py') }}
+          restore-keys: |
+            ${{ runner.os }}-pip-
+      - name: Install pretalx
+        run: pip3 install pretalx
+      - name: Install Dependencies
+        run: pip3 install black -Ue .
+      - name: Run black
+        run: black --check .
+        working-directory: .
   docformatter:
     name: docformatter
     runs-on: ubuntu-latest