Replace mergify with action (#619)

.github/workflows/merge.yml

@@ -0,0 +1,17 @@
+name: PR
+
+on:
+  pull_request:
+    types: [labeled]
+
+jobs:
+  auto-merge:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: jacogr/action-merge@master
+        with:
+          checks: build,lint,test
+          labels: automerge
+          strategy: squash
+          token: ${{ secrets.GH_PAT_BOT }}