Add mergify config for release 2021.03

.mergify.yml

@@ -141,6 +141,35 @@ pull_request_rules:
         strict_method: rebase
         method: squash
       delete_head_branch: {}
+  - name: Merge queue for release-2021.03
+    conditions:
+      - base=release-2021.03
+      - -conflict
+      - "#approved-reviews-by>=1"
+      - "#changes-requested-reviews-by=0"
+      - status-success="Travis CI - Pull Request"
+      - status-success="codecov/patch"
+      - label=ready-to-merge
+    actions:
+      merge:
+        strict: smart
+        strict_method: rebase
+        method: squash
+      delete_head_branch: { }
+  - name: Merge queue for release-2021.03 (automatic backports)
+    conditions:
+      - base=release-2021.03
+      - head~=^mergify/bp/release-2021.03/pr.*
+      - -conflict
+      - "#changes-requested-reviews-by=0"
+      - status-success="Travis CI - Pull Request"
+      - status-success="codecov/patch"
+    actions:
+      merge:
+        strict: smart
+        strict_method: rebase
+        method: squash
+      delete_head_branch: { }
   - name: Create backports for v2019.10
     conditions:
       - base=master
@@ -159,3 +188,12 @@ pull_request_rules:
       backport:
         branches:
           - release-2020.06
+  - name: Create backports for v2021.03
+    conditions:
+      - base=master
+      - merged
+      - label=backport-to-2021.03
+    actions:
+      backport:
+        branches:
+          - release-2021.03