Update tox to remove python35

requirements/edx/django31.txt

@@ -0,0 +1 @@
+Django>=3.1,<3.2

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{35}-django{22},py{38}-django{22,30}
+envlist = py{38}-django{22,30,31}
 
 # This is needed to prevent the lms, cms, and openedx packages inside the "Open
 # edX" package (defined in setup.py) from getting installed into site-packages
@@ -70,6 +70,7 @@ passenv =
 deps =
     django22: -r requirements/edx/django.txt
     django30: -r requirements/edx/django30.txt
+    django31: -r requirements/edx/django31.txt
     -r requirements/edx/testing.txt
 whitelist_externals =
     /bin/bash