Add bot to enforce hold PRs aren't merged (#456)

* Add bot to enforce hold PRs aren't merged

* Use commit hash rather than tag

.github/workflows/enforce-labels.yml

@@ -0,0 +1,13 @@
+name: Enforce PR labels
+
+on:
+  pull_request:
+    types: [labeled, unlabeled, opened, edited, synchronize]
+jobs:
+  enforce-label:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: yogevbd/enforce-label-action@8d1e1709b1011e6d90400a0e6cf7c0b77aa5efeb
+      with:
+        BANNED_LABELS: "hold"
+        BANNED_LABELS_DESCRIPTION: "PRs on hold cannot be merged"