github: enable flake8 postsubmit testing

Change-Id: I8532f52b3016eb491ddeb48463459d74afd36015
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/354514
Reviewed-by: Joanna Wang <jojwang@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

.github/workflows/flake8-postsubmit.yml

@@ -0,0 +1,23 @@
+# GitHub actions workflow.
+# https://help.github.com/en/actions/automating-your-workflow-with-github-actions/workflow-syntax-for-github-actions
+# https://github.com/marketplace/actions/python-flake8
+
+name: Flake8
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  lint:
+    name: Python Lint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-python@v4
+        with:
+          python-version: "3.9"
+      - name: Run flake8
+        uses: julianwachholz/flake8-action@v2
+        with:
+          checkName: "Python Lint"