Add black/isort action

Author: timj

.github/workflows/formatting.yaml

@@ -0,0 +1,27 @@
+name: check Python formatting
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Set up Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.8
+
+      - name: Install isort and black
+        run: pip install isort black
+
+      - name: Run isort
+        run: isort --check-only python/ tests/
+
+      - name: Run black
+        run: black --check --verbose --diff python/ tests/