(actions) add black linting

.github/workflows/lint.yml

@@ -0,0 +1,14 @@
+name: Lint
+
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          submodules: recursive
+      - uses: psf/black@stable
+        with:
+          options: --check --diff --color -l 120 --exclude docs