Automatically merge known dependencies updated with Dependabot

This ought to be safe enough, esp. in a template repo.

.github/workflows/dependabot-automerge.yaml

@@ -0,0 +1,24 @@
+name: Dependabot auto-merge
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: github.actor == 'dependabot[bot]'
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@v2
+        with:
+          github-token: "${{ secrets.GITHUB_TOKEN }}"
+      - name: Enable auto-merge for Dependabot PRs
+        # TODO: put this list somewhere more manageable
+        if: contains(fromJSON('["ruff", "mypy", "pytest", "ansi2html", "lxml", "jinja2"]'), steps.metadata.outputs.dependency-names) && steps.metadata.outputs.update-type == 'version-update:semver-patch'
+        run: gh pr merge --auto --merge "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GH_TOKEN: ${{secrets.GITHUB_TOKEN}}