[CI] Add stalebot (#481)

* Add stalebot

* style

* Remove the closing logic

* Make sure not to spam

.github/workflows/stale.yml

+name: Stale Bot
+
+on:
+  schedule:
+    - cron: "0 15 * * *"
+
+jobs:
+  close_stale_issues:
+    name: Close Stale Issues
+    if: github.repository == 'huggingface/diffusers'
+    runs-on: ubuntu-latest
+    env:
+      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Setup Python
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.7
+
+    - name: Install requirements
+      run: |
+        pip install PyGithub
+    - name: Close stale issues
+      run: |
+        python utils/stale.py

utils/stale.py

+# Copyright 2022 The HuggingFace Team, the AllenNLP library authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""
+Script to close stale issue. Taken in part from the AllenNLP repository.
+https://github.com/allenai/allennlp.
+"""
+import os
+from datetime import datetime as dt
+
+from github import Github
+
+
+LABELS_TO_EXEMPT = [
+    "good first issue",
+    "good second issue",
+    "good difficult issue",
+    "enhancement",
+    "new pipeline/model",
+    "new scheduler",
+    "wip",
+]
+
+
+def main():
+    g = Github(os.environ["GITHUB_TOKEN"])
+    repo = g.get_repo("huggingface/diffusers")
+    open_issues = repo.get_issues(state="open")
+
+    for issue in open_issues:
+        comments = sorted([comment for comment in issue.get_comments()], key=lambda i: i.created_at, reverse=True)
+        last_comment = comments[0] if len(comments) > 0 else None
+        if (
+            last_comment is not None
+            and last_comment.user.login != "github-actions[bot]"
+            and (dt.utcnow() - issue.updated_at).days > 23
+            and (dt.utcnow() - issue.created_at).days >= 30
+            and not any(label.name.lower() in LABELS_TO_EXEMPT for label in issue.get_labels())
+        ):
+            issue.create_comment(
+                "This issue has been automatically marked as stale because it has not had "
+                "recent activity. If you think this still needs to be addressed "
+                "please comment on this thread.\n\nPlease note that issues that do not follow the "
+                "[contributing guidelines](https://github.com/huggingface/diffusers/blob/main/CONTRIBUTING.md) "
+                "are likely to be ignored."
+            )
+            issue.edit(labels=["stale"])
+
+
+if __name__ == "__main__":
+    main()