Add additional test env for tox covering django3

tox.ini

@@ -4,12 +4,14 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py3-django2,flake8
+envlist = py3-django{2,3},flake8
 
 [testenv]
 extras = testing
 
 deps = coverage
+    django2: Django >= 2.2.24, < 3
+    django3: Django >= 3.2.7, <4
 
 setenv =
     DJANGO_SETTINGS_MODULE=debexpo.settings.test