Add stale issue bot (dmlc#3673)

* Create stale.yml

* Update stale.yml

.github/workflows/stale.yml

@@ -0,0 +1,31 @@
+# This workflow warns and then closes issues and PRs that have had no activity for a specified amount of time.
+#
+# You can adjust the behavior by modifying this file.
+# For more information, see:
+# https://github.com/actions/stale
+name: Mark stale issues and pull requests
+
+on:
+  schedule:
+  - cron: '0 1 * * *'
+
+jobs:
+  stale:
+
+    runs-on: ubuntu-latest
+    permissions:
+      issues: write
+      pull-requests: write
+
+    steps:
+    - uses: actions/[email protected]
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        days-before-issue-stale: 30
+        days-before-issue-close: 7
+        days-before-pr-stale: -1 # disable stale bot on pr
+        days-before-pr-close: -1 # disable stale bot on pr
+        stale-issue-message: 'This issue has been automatically marked as stale due to lack of activity. It will be closed if no further activity occurs. Thank you'
+        close-issue-message: 'This issue is closed due to lack of activity. Feel free to reopen it if you still have questions.'
+        stale-issue-label: 'stale-issue'
+        exempt-issue-labels: 'RFC,pinned,bug,dist DGL,doc,enhancement,feature request,help wanted,model request,wontfix,Suspended,system performance,windows,PyTorch,MXNet,TensorFlow'