          else:
              return []
  
      def _check_upload_to(self):
          if isinstance(self.upload_to, six.string_types) and self.upload_to.startswith('/'):
              return [
                  checks.Error(
                      "%s's 'upload_to' argument must be a relative path, not an "
                      "absolute path." % self.__class__.__name__,
                      obj=self,
 
</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             if isinstance(bcc, six.string_types):
 </s> add             if isinstance(bcc, str): </s> remove             if isinstance(cc, six.string_types):
 </s> add             if isinstance(cc, 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(reply_to, six.string_types):
 </s> add             if isinstance(reply_to, str):
        if isinstance(self.upload_to, str) and self.upload_to.startswith('/'):
          else:
              return []
  
      def _check_upload_to(self):
        if isinstance(self.upload_to, str) and self.upload_to.startswith('/'):
              return [
                  checks.Error(
                      "%s's 'upload_to' argument must be a relative path, not an "
                      "absolute path." % self.__class__.__name__,
                      obj=self,
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/files.py