update test matrix in tox (GitHub Actions can be done later)

Author: vdboor

tox.ini

@@ -1,9 +1,9 @@
 [tox]
 envlist =
-	py35-django{21,22}
-	py36-django{21,22,30,31}
-	py37-django{21,22,30,31}
-	py38-django{21,22,30,31}
+	py36-django{22,30,31}
+	py37-django{22,30,31}
+	py38-django{22,30,31,32}
+	py39-django{40}
 	docs
 
 [testenv]
@@ -12,14 +12,14 @@ setenv =
 	postgres: DEFAULT_DATABASE = postgres:///default
 	postgres: SECONDARY_DATABASE = postgres:///secondary
 deps =
-	ipdb
 	coverage
 	dj-database-url
 	django21: Django ~= 2.1
 	django22: Django ~= 2.2
 	django30: Django ~= 3.0
 	django31: Django ~= 3.1
-	djangomaster: https://github.com/django/django/archive/master.tar.gz
+	django40: Django==4.0b1
+	djangomain: https://github.com/django/django/archive/main.tar.gz
 	postgres: psycopg2
 commands =
 	coverage run --source polymorphic runtests.py