enforce labels on PRs (jupyter#6235)

Author: blink1073

.github/workflows/enforce-label.yml

@@ -0,0 +1,11 @@
+name: Enforce PR label
+
+on:
+  pull_request:
+    types: [labeled, unlabeled, opened, edited, synchronize]
+jobs:
+  enforce-label:
+    runs-on: ubuntu-latest
+    steps:
+      - name: enforce-triage-label
+        uses: jupyterlab/maintainer-tools/.github/actions/enforce-label@v1