Maintain labels by the way of declarative configuration.

.github/workflows/labels_sync.yaml

@@ -0,0 +1,13 @@
+name: Sync labels
+on: issues
+
+jobs:
+
+  sync_labels:
+    name: Sync labels
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2.3.1
+    - uses: lannonbr/issue-label-manager-action@2.0.0
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}