Fix bug with graph_models in python 3
Fix bug with graph_models in python 3

Fixes issue #799
I don't know if this needs a test or if it's fine.
jpadilla/django-extensions,django-extensions/django-extensions,kevgathuku/django-extensions,kevgathuku/django-extensions,linuxmaniac/django-extensions,haakenlid/django-extensions,linuxmaniac/django-extensions,jpadilla/django-extensions,django-extensions/django-extensions,django-extensions/django-extensions,jpadilla/django-extensions,haakenlid/django-extensions,haakenlid/django-extensions,kevgathuku/django-extensions,linuxmaniac/django-extensions