Github: auto close stale PRs

https://github.com/apps/stale

.github/CONTRIBUTING.md

@@ -55,3 +55,7 @@ Failing to include __new__ dependencies will fail the CI tests.
 
 If by some mistake you end up with multiple commits in your PR and one of our developers asks you to squash your commits please do __NOT__ create a new pull request.
 Instead please follow [this rebase tutorial](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History#Changing-Multiple-Commit-Messages).
+
+### Pull Request (PR) max age
+
+Pull Requests that have not been updated in the last 180 days will automatically be labeled S-stale. After 7 days of additional inactivity the PR will automatically be closed except if one of the Alpine developers will label it with S-WIP (Work In Progress).

.github/stale.yml

+# Configuration for probot-stale - https://github.com/probot/stale
+
+# Number of days of inactivity before an Issue or Pull Request becomes stale
+daysUntilStale: 180
+
+# Number of days of inactivity before a stale Issue or Pull Request is closed.
+# Set to false to disable. If disabled, issues still need to be closed manually, but will remain marked as stale.
+daysUntilClose: 7
+
+# Issues or Pull Requests with these labels will never be considered stale. Set to `[]` to disable
+exemptLabels:
+  - S-WIP
+
+# Set to true to ignore issues in a project (defaults to false)
+exemptProjects: false
+
+# Set to true to ignore issues in a milestone (defaults to false)
+exemptMilestones: false
+
+# Label to use when marking as stale
+staleLabel: S-stale
+
+# Comment to post when marking as stale. Set to `false` to disable
+markComment: >
+  This issue has been automatically marked as stale because it has not had
+  recent activity. It will be closed if no further activity occurs. Thank you
+  for your contributions.
+
+# Comment to post when removing the stale label.
+# unmarkComment: >
+#   Your comment here.
+
+# Comment to post when closing a stale Issue or Pull Request.
+# closeComment: >
+#   Your comment here.
+
+# Limit the number of actions per hour, from 1-30. Default is 30
+limitPerRun: 30
+
+# Limit to only `issues` or `pulls`
+only: pulls
+
+# Optionally, specify configuration settings that are specific to just 'issues' or 'pulls':
+pulls:
+  daysUntilStale: 180
+  markComment: >
+    This pull request has been automatically marked as stale because it has not
+    had activity in the last 180 days. It will be closed in 7 days if no further
+    activity occurs. Thank you for your contributions.
+
+# issues:
+#   exemptLabels:
+#     - confirmed