Add a black (code formatting) github action on PR and push

.github/workflows/black.yml

@@ -0,0 +1,14 @@
+name: Lint
+
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: psf/black@stable
+        with:
+          options: "--check --verbose"
+          src: ". vppcfg"
+