Adding "stale" GitHub Action (GH-21247)

Adding "stale" GitHub Action

Added the "stale" GitHub action to the CPython repo.
PR's older than 30 days will be labeled as stale using the "stale-pr" label.

Closes https://github.com/python/core-workflow/issues/372

Co-authored-by: Brett Cannon <brett@python.org>
diff --git a/.github/workflows/stale.yml b/.github/workflows/stale.yml
new file mode 100644
index 0000000..706d8e1
--- /dev/null
+++ b/.github/workflows/stale.yml
@@ -0,0 +1,19 @@
+name: Mark 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 PR is stale because it has been open for 30 days with no activity. Remove stale label or comment or this will be closed in 5 days'
+        stale-pr-label: 'stale'
+        days-before-stale: 30
+        days-before-close: -1