github: add black check action

Change-Id: Ic87c1c5c72fb8a01108146c1f9d78466acb57278
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/540021
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

.github/workflows/black.yml

@@ -0,0 +1,16 @@
+# GitHub actions workflow.
+# https://help.github.com/en/actions/automating-your-workflow-with-github-actions/workflow-syntax-for-github-actions
+# https://black.readthedocs.io/en/stable/integrations/github_actions.html
+
+name: Format
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  format:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+      - uses: psf/black@stable