Add stale bot

A lot of issues and PRs are outdated. This bot helps to close them.

.github/workflows/stale.yml

@@ -0,0 +1,17 @@
+name: Mark stale issues and pull requests
+
+on:
+  schedule:
+  - cron: "0 0 * * *"
+
+jobs:
+  stale:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/stale@v1
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        stale-issue-message: 'This issue has not seen any activity in a long time. If the issue descriped still exists in recent versions of Alchemy, please open a new issue or preferably open a PR with a fix. Thanks for reporting.'
+        stale-pr-message: 'This pull request has not seen any activiy in a long time. Probably because of missing tests or a necessary rebase. Please open a new PR to latest master if you want to continue working on this. Thanks for the contribution.'