Add configuration for stale bot (#454)

Ideally we would have the resources to address and resolve all issues, and review and merge all pull requests. But we don't.
Rather than leave these open indefinitely, it's better to set user expectations that these will just age out.

.github/workflows/stale.yml

diff --git a/.github/workflows/stale.yml b/.github/workflows/stale.yml
new file mode 100644
index 0000000..816ce1d
--- /dev/null
+++ b/.github/workflows/stale.yml
@@ -0,0 +1,59 @@
+# See https://github.com/marketplace/actions/close-stale-issues
+
+name: Mark stale issues and pull requests
+
+on:
+  schedule:
+  # run at 22:45 UTC daily
+  - cron: "45 22 * * *"
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/stale@v3
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+
+        # NB: We start with very long duration while trimming existing issues,
+        # with the hope to reduce when/if we get better at keeping up with user support.
+
+        # The number of days old an issue can be before marking it stale.
+        days-before-stale: 180
+        # Number of days of inactivity before a stale issue is closed
+        days-before-close: 30
+
+        # If an issue/PR is assigned, trust the assignee to stay involved
+        # Can revisit if these get stale
+        exempt-all-assignees: true
+        # Issues with these labels will never be considered stale
+        exempt-issue-labels: "need: discussion,cleanup"
+
+        # Label to use when marking an issue as stale
+        stale-issue-label: 'Can Close?'
+        stale-pr-label: 'Can Close?'
+
+        stale-issue-message: >
+          This issue has been automatically marked as stale because it has not had
+          any activity for 180 days.
+          It will be closed if no further activity occurs in 30 days.
+
+          Collaborators can add an assignee to keep this open indefinitely.
+          Thanks for your contributions to rules_python!
+
+        stale-pr-message: >
+          This Pull Request has been automatically marked as stale because it has not had
+          any activity for 180 days.
+          It will be closed if no further activity occurs in 30 days.
+
+          Collaborators can add an assignee to keep this open indefinitely.
+          Thanks for your contributions to rules_python!
+
+        close-issue-message: >
+          This issue was automatically closed because it went 30 days without a reply
+          since it was labeled "Can Close?"
+
+        close-pr-message: >
+          This PR was automatically closed because it went 30 days without a reply
+          since it was labeled "Can Close?"