Router throws AttributeError when project urls.py follows django 2 style urls.

[daanvdk]

In Django 2 you import url functions from django.urls instead of django.conf.urls.
However since binder.router only imports django and then uses django.conf.urls this throws an AttributeError when the url file does not import django.conf.urls itself. This used to always be the case since this is how old projects configured urls which is probably why this slipped through. For now just importing django.conf.urls in the project fixes the issue but probably should be fixed in binder itself.