CI: using milestone action to add it on merge

Author: bsipocz

.github/workflows/milestone.yml

@@ -0,0 +1,18 @@
+name: Milestone -- Add latest if missing on merge
+
+on:
+  pull_request_target:
+    types:
+      - closed
+    branches:
+      - 'main'
+
+jobs:
+  milestone_pr:
+    name: attach missing milestone to PR
+    if: github.event.pull_request.merged == true
+    runs-on: ubuntu-latest
+    steps:
+      - uses: scientific-python/attach-next-milestone-action@a4889cfde7d2578c1bc7400480d93910d2dd34f6
+        env:
+          GITHUB_TOKEN:: ${{ secrets.GITHUB_TOKEN }}