Added Django 4.0 to test matrix.

Author: carltongibson

tox.ini

@@ -3,14 +3,15 @@ envlist =
        {py35,py36,py37}-django22,
        {py36,py37,py38,py39}-django31,
        {py36,py37,py38,py39}-django32,
-       {py38,py39}-djangomain,
+       {py38,py39}-{django40,djangomain},
        base,dist,docs,
 
 [travis:env]
 DJANGO =
     2.2: django22
     3.1: django31
     3.2: django32
+    4.0: django40
     main: djangomain
 
 [testenv]
@@ -23,6 +24,7 @@ deps =
         django22: Django>=2.2,<3.0
         django31: Django>=3.1,<3.2
         django32: Django>=3.2,<4.0
+        django40: Django>=4.0a1,<5.0
         djangomain: https://github.com/django/django/archive/main.tar.gz
         -rrequirements/requirements-testing.txt
         -rrequirements/requirements-optionals.txt