add CI workflow to auto-merge dependency upgrades

...only patch and minor versions, and only if CI passes.

https://docs.github.com/en/code-security/dependabot/working-with-dependabot/automating-dependabot-with-github-actions#enable-auto-merge-on-a-pull-request

.github/workflows/auto-merge-dependabot.yaml

@@ -0,0 +1,30 @@
+# Auto-merge Dependabot PRs that upgrade patch or minor versions if CI passes
+# Copied from https://docs.github.com/en/code-security/dependabot/working-with-dependabot/automating-dependabot-with-github-actions#enable-auto-merge-on-a-pull-request
+# Also see https://github.com/dependabot/fetch-metadata
+
+name: Dependabot auto-merge
+on: pull_request
+
+permissions:
+  pull-requests: write
+  contents: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: github.actor == 'dependabot[bot]'
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@v1.1.1
+        with:
+          github-token: "${{ secrets.GITHUB_TOKEN }}"
+
+      - name: Enable auto-merge for Dependabot PRs
+        if: >
+          ! contains(steps.metadata.outputs.dependency-names, 'tlslite-ng') &&
+          steps.metadata.outputs.update-type != 'version-update:semver-major'
+        run: gh pr merge --auto --rebase "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}