Update Django test matrix

Author: nikmolnar

.github/workflows/python-package.yml

@@ -16,7 +16,7 @@ jobs:
       matrix:
         python-version: [3.6, 3.7, 3.8, 3.9]
         celery-version: [4.*, 5.*]
-        django-version: [2.2, 3.1, 3.2]
+        django-version: [2.2, 3.2, 4.0]
 
     steps:
     - uses: actions/checkout@v2