Fix tox matrix

tox.ini

@@ -1,18 +1,17 @@
 [tox]
 envlist =
-    {py36,py37,py38,py39}-django-22
-    {py36,py37,py38,py39}-django-30
-    {py36,py37,py38,py39}-django-31
-    {py36,py37,py38,py39}-django-master
+    py{36,37,38,39}-django-22
+    py{36,37,38,39}-django-30
+    py{36,37,38,39}-django-31
+    py{36,37,38,39}-django-master
 
 [testenv]
-install_command = pip install --pre {opts} {packages}
-setenv =
-    PYTHONPATH = {toxinidir}:{toxinidir}/social_django
-commands = coverage run manage.py test
+passenv = *
+commands =
+    coverage run manage.py test
 deps =
-    django-22: Django>=2.2,<2.3
-    django-30: Django>=3.0,<3.1
-    django-31: Django>=3.1,<3.2
-    django-master: https://github.com/django/django/archive/master.tar.gz
+    py{36,37,38,39}-django-22: Django>=2.2,<2.3
+    py{36,37,38,39}-django-30: Django>=3.0,<3.1
+    py{36,37,38,39}-django-31: Django>=3.1,<3.2
+    py{36,37,38,39}-django-master: https://github.com/django/django/archive/master.tar.gz
     -r{toxinidir}/dev-requirements.txt