Add official Django 3.1 support

Author: codingjoe

.github/workflows/ci.yml

@@ -54,6 +54,7 @@ jobs:
         django-version:
           - 2.2.*
           - 3.0.*
+          - 3.1.*
     steps:
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v2