CI: Do not allow merge if labelled DO-NOT-MERGE (pythonGH-103337)

(cherry picked from commit 090e26e)

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>
Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>

.github/workflows/require-pr-label.yml

@@ -0,0 +1,17 @@
+name: Check labels
+
+on:
+  pull_request:
+    types: [opened, reopened, labeled, unlabeled, synchronize]
+
+jobs:
+  label:
+    name: DO-NOT-MERGE
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: mheap/github-action-required-labels@v4
+        with:
+          mode: exactly
+          count: 0
+          labels: "DO-NOT-MERGE"