          return errors
  
      def _check_relation_model_exists(self):
          rel_is_missing = self.remote_field.model not in self.opts.apps.get_models()
          rel_is_string = isinstance(self.remote_field.model, six.string_types)
          model_name = self.remote_field.model if rel_is_string else self.remote_field.model._meta.object_name
          if rel_is_missing and (rel_is_string or not self.remote_field.model._meta.swapped):
              return [
                  checks.Error(
                      "Field defines a relation with model '%s', which is either "
 
</s> remove         rel_is_string = isinstance(self.remote_field.model, six.string_types)
 </s> add         rel_is_string = isinstance(self.remote_field.model, str) </s> remove                 not isinstance(self.remote_field.model, six.string_types) and
 </s> add                 not isinstance(self.remote_field.model, str) and </s> remove         if isinstance(self.remote_field.through, six.string_types) or not self.remote_field.through._meta.managed:
 </s> add         if isinstance(self.remote_field.through, str) or not self.remote_field.through._meta.managed: </s> remove             if isinstance(self.remote_field.model, six.string_types):
 </s> add             if isinstance(self.remote_field.model, str): </s> remove         if isinstance(self.remote_field.model, six.string_types):
 </s> add         if isinstance(self.remote_field.model, str):
        rel_is_string = isinstance(self.remote_field.model, str)
          return errors
  
      def _check_relation_model_exists(self):
          rel_is_missing = self.remote_field.model not in self.opts.apps.get_models()
        rel_is_string = isinstance(self.remote_field.model, str)
          model_name = self.remote_field.model if rel_is_string else self.remote_field.model._meta.object_name
          if rel_is_missing and (rel_is_string or not self.remote_field.model._meta.swapped):
              return [
                  checks.Error(
                      "Field defines a relation with model '%s', which is either "
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/fields/related.py