Add action to automatically apply Black (#30)

.github/workflows/format-pr.yml

@@ -0,0 +1,36 @@
+name: Format PR
+
+on:
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  format:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Configure git
+      run: |
+        git config user.name "Github Actions Bot"
+        git config user.email "<>"
+    - name: Set up Python 3.8
+      uses: actions/setup-python@v2
+      with:
+        python-version: "3.8"
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install black
+    - name: Format with Black
+      run: |
+        black .
+    - name: Commit formatted code
+      run: |
+        git update-index --refresh
+        git add .
+        if ! git diff-index --quiet HEAD -- ; then
+          git commit -m "Format with Black"
+          git push
+        fi