Attempt to correct Mergify config for cascaded backports

Author: HoloRin

.github/mergify.yml

@@ -17,12 +17,26 @@ pull_request_rules:
     conditions:
       - base=master
       - label=backport-v3.9.x
+      - label!=backport-v3.8.x
     actions:
       backport:
         branches:
           - v3.9.x
         assignees:
           - "{{ author }}"
+  - name: Automatically backport to v3.9.x & v3.8.x based on labels
+    conditions:
+      - base=master
+      - label=backport-v3.9.x
+      - label=backport-v3.8.x
+    actions:
+      backport:
+        branches:
+          - v3.9.x
+        labels:
+          - backport-v3.8.x
+        assignees:
+          - "{{ author }}"
   - name: Automatically backport to v3.8.x based on label
     conditions:
       - base=v3.9.x