Add check for release notes (#2569)

Co-authored-by: Isaac Virshup <ivirshup@gmail.com>

.github/pull_request_template.md

@@ -2,3 +2,9 @@
 Thanks for opening a PR to scanpy!
 Please be sure to follow the guidelines in our contribution guide (https://scanpy.readthedocs.io/en/latest/dev/index.html) to familiarize yourself with our workflow and speed up review.
 -->
+
+<!-- Please check (“- [x]”) and fill in the following boxes -->
+- [ ] Closes #
+- [ ] Tests included or not required because:
+<!-- Only check the following box if you did not include release notes -->
+- [ ] Release notes not necessary because:

.github/workflows/check-pr.yml

@@ -19,13 +19,41 @@ on:
       # code change (e.g. this workflow)
       - synchronize
 
-env:
-  LABELS: ${{ join( github.event.pull_request.labels.*.name, '|' ) }}
-
 jobs:
+  # This job verifies that the milestone is present or not necessary
+  # and determines if “check-relnotes” needs to be run.
   check-milestone:
     name: Check Milestone
     runs-on: ubuntu-latest
     steps:
-      - if: github.event.pull_request.milestone == null && contains( env.LABELS, 'no milestone' ) == false
-        run: exit 1
+      - name: Check if milestone or “no milestone” label is present
+        uses: flying-sheep/check@v1
+        with:
+          success: ${{ github.event.pull_request.milestone != null || contains(github.event.pull_request.labels.*.name, 'no milestone') }}
+      - name: Check if the “Release notes” checkbox is checked and filled
+        uses: kaisugi/action-regex-match@v1.0.0
+        id: checked-relnotes
+        with:
+          text: ${{ github.event.pull_request.body }}
+          regex: '^\s*- \[x\] Release notes not necessary because:(.*)$'
+          flags: m
+    outputs:
+      no-relnotes-reason: ${{ steps.checked-relnotes.outputs.group1 }}
+  # This job verifies that the relevant release notes file has been modified.
+  check-relnotes:
+    name: Check for release notes
+    runs-on: ubuntu-latest
+    needs: check-milestone
+    if: ${{ needs.check-milestone.outputs.no-relnotes-reason == '' && !contains(github.event.pull_request.labels.*.name, 'Development Process 🚀') }}
+    steps:
+      - uses: actions/checkout@v3
+      - name: Find out if relevant release notes are modified
+        uses: dorny/paths-filter@v2
+        id: changes
+        with:
+          filters: | # this is intentionally a string
+            relnotes: 'docs/release-notes/${{ github.event.pull_request.milestone.title }}.md'
+      - name: Check if relevant release notes are modified
+        uses: flying-sheep/check@v1
+        with:
+          success: ${{ steps.changes.outputs.relnotes }}