use six.string_types for python3

Author: mrj0

django/db/models/fields/related.py

@@ -55,7 +55,7 @@ class MyModel(Model):
  else:
  # Look for an "app.Model" relation
 
- if isinstance(relation, basestring):
+ if isinstance(relation, six.string_types):
  try:
  app_label, model_name = relation.split(".")
  except ValueError: