add a stale bot (#134)

.github/workflows/stale.yml

@@ -0,0 +1,18 @@
+name: "Close stale pull requests"
+on:
+  schedule:
+    - cron: "0 0 * * *"
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v3
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          stale-pr-message: "This pull request has been automatically marked as stale because it has not had
+    recent activity. It will be closed if no further activity occurs. Thank you
+    for your contributions."
+          days-before-stale: 30
+          days-before-close: 6
+          exempt-pr-labels: "pinned, proposal"