Test on Django 4

Author: PeterJCLaw

.circleci/config.yml

@@ -35,7 +35,7 @@ references:
     matrix:
       parameters:
         python-version: ["3.5.5", "3.6", "3.7", "3.8", "3.9", "3.10"]
-        django-version: ["2.2", "3.0", "3.1", "3.2"]
+        django-version: ["2.2", "3.0", "3.1", "3.2", "4.0"]
       exclude:
         - python-version: "3.5.5"
           django-version: "3.0"
@@ -44,6 +44,13 @@ references:
         - python-version: "3.5.5"
           django-version: "3.2"
 
+        - python-version: "3.5.5"
+          django-version: "4.0"
+        - python-version: "3.6"
+          django-version: "4.0"
+        - python-version: "3.7"
+          django-version: "4.0"
+
 executors:
   python:
     parameters: