Add action for Dependabot:

cf. https://github.com/yasslab/dependabot_auto_merge

Author: yasulab

.github/workflows/dependabot.yml

@@ -0,0 +1,19 @@
+name: Auto-merge Dependabot PRs
+on:
+  pull_request:
+  # Allows you to run this workflow manually from the Actions tab
+  # https://docs.github.com/en/actions/managing-workflow-runs/manually-running-a-workflow
+  workflow_dispatch:
+
+permissions:
+  contents:      write
+  pull-requests: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: github.actor == 'dependabot[bot]'
+    steps:
+      - uses: yasslab/dependabot_auto_merge@main
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}