Add official Django 3.1 support

Author: codingjoe

.github/workflows/ci.yml

@@ -36,8 +36,9 @@ jobs:
       matrix:
         python-version: [3.8]
         django-version:
-          - "2.2"
-          - "3.0"
+          - "2.2.*"
+          - "3.0.*"
+          - "3.1.*"
     services:
       postgres:
         image: postgres
@@ -53,7 +54,7 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
       - uses: actions/checkout@v2.0.0
-      - run: python -m pip install psycopg2-binary Django~=${{ matrix.django-version }}
+      - run: python -m pip install psycopg2-binary Django==${{ matrix.django-version }}
       - run: python setup.py test
         env:
           DB_PORT: ${{ job.services.postgres.ports[5432] }}