fix: Stop support django dev

Author: untitaker

tox.ini

@@ -10,7 +10,6 @@ envlist =
     pypy
     flake8
     # contrib
-    {py37}-django-dev
     {py35,py36,py37}-django-{200}
     {py27,py35}-django-111
     {py27,py34,py35,py36}-django-{18,19,110}
@@ -37,7 +36,6 @@ deps =
     django-110: Django>=1.10,<1.11
     django-111: Django>=1.11,<1.12
     django-200: Django>=2.0,<2.1
-    django-dev: git+https://github.com/django/django.git#egg=Django
     flask-10: Flask>=0.10,<0.11
     flask-11: Flask>=0.11,<0.12
     flask-12: Flask>=0.12,<0.13