Add GH Action for black

Author: unode

.github/workflows/format.yml

@@ -0,0 +1,18 @@
+name: black-action
+on: [push, pull_request]
+jobs:
+  linter_name:
+    name: Run black formatter
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Check files using the black formatter
+        uses: rickstaa/action-black@v1
+        id: action_black
+        with:
+          black_args: "polypomo"
+      - name: Annotate diff changes using reviewdog
+        if: steps.action_black.outputs.is_formatted == 'true'
+        uses: reviewdog/action-suggester@v1
+        with:
+          tool_name: blackfmt