tox.ini: allow for pytest 3.5 (#588)

Author: blueyed

tox.ini

@@ -8,7 +8,7 @@ envlist =
 
 [testenv]
 deps =
-    pytest>=3.0,<3.5
+    pytest>=3.0,<3.6
     django-configurations==2.0
     pytest-xdist==1.15
     {env:_PYTESTDJANGO_TOX_EXTRA_DEPS:}