remove django deprecated version

Author: rednaks

tox.ini

@@ -1,7 +1,6 @@
 [tox]
 isolated_build = True
 envlist =
-    {py3}-django-31
     {py3}-django-32
 
 [tox.package]
@@ -11,7 +10,6 @@ setenv =
     PYTHONPATH = {toxinidir}:{toxinidir}/django_async_orm
 commands = coverage run --source django_async_orm manage.py test -v2 {posargs}
 deps =
-    django-31: Django>=3.1,<3.2
     django-32: Django>=3.2,<4.0
     -r{toxinidir}/tests/requirements.txt