Test formatting on GHaction.

.github/workflows/python-package.yml

@@ -0,0 +1,38 b''
+# This workflow will install Python dependencies, run tests and lint with a variety of Python versions
+# For more information see: https://help.github.com/actions/language-and-framework-guides/using-python-with-github-actions
+
+name: Python package
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: [3.8]
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v2
+      with:
+        python-version: ${{ matrix.python-version }}
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install darker
+    - name: Lint with darker
+      run: |
+        git fetch
+        darker -r 60625f241f298b5039cb2debc365db38aa7bb522 --check --diff . || (
+        echo "Changes need auto-formatting. Run:"
+        echo "    darker -r 60625f241f298b5039cb2debc365db38aa7bb522"
+        echo "then commit and push changes to fix."
+        exit 1
+        )