Expanded test matrix

.github/workflows/code.yaml

@@ -15,7 +15,7 @@ jobs:
       max-parallel: 4
       matrix:
         python-version: [3.6, 3.7, 3.8]
-        django-version: ["~=2.0.0", "~=2.1.0", "~=2.2.0", "~=3.0.0"]
+        django-version: ["~=2.0.0", "~=2.1.0", "~=2.2.0", "~=3.0.0", "~=3.1.0"]
     services:
       postgres:
         image: postgres:latest