      """
      try:
          if isinstance(model, tuple):
              model_tuple = model
          elif isinstance(model, six.string_types):
              app_label, model_name = model.split(".")
              model_tuple = app_label, model_name.lower()
          else:
              model_tuple = model._meta.app_label, model._meta.model_name
          assert len(model_tuple) == 2
 
</s> remove                 default = six.text_type()
 </s> add                 default = str() </s> remove         elif f.geom_type == 'RASTER' and isinstance(value, six.string_types):
 </s> add         elif f.geom_type == 'RASTER' and isinstance(value, str): </s> remove         elif isinstance(srs_input, six.string_types):
 </s> add         elif isinstance(srs_input, str): </s> remove                 default = six.binary_type()
 </s> add                 default = bytes() </s> remove         rel_is_string = isinstance(self.remote_field.model, six.string_types)
 </s> add         rel_is_string = isinstance(self.remote_field.model, str)
        elif isinstance(model, str):
      """
      try:
          if isinstance(model, tuple):
              model_tuple = model
        elif isinstance(model, str):
              app_label, model_name = model.split(".")
              model_tuple = app_label, model_name.lower()
          else:
              model_tuple = model._meta.app_label, model._meta.model_name
          assert len(model_tuple) == 2
keep keep keep keep replace keep keep keep keep keep
 Refs #23919 -- Removed six.<various>_types usage

Thanks Tim Graham and Simon Charette for the reviews. 
django/db/models/utils.py