Add official Django 3.1 support

Author: codingjoe

.github/workflows/ci.yml

@@ -48,6 +48,7 @@ jobs:
         django-version:
           - "2.2"
           - "3.0"
+          - "3.1"
     runs-on: ubuntu-latest
     steps:
       - name: Set up Python ${{ matrix.python-version }}