      def __init__(self):
          super(TeamField, self).__init__(max_length=100)
  
      def get_db_prep_save(self, value, connection):
          return six.text_type(value.title)
  
      def to_python(self, value):
          if isinstance(value, Team):
              return value
          return Team(value)
 
</s> remove         if isinstance(value, six.string_types) or value is None:
 </s> add         if isinstance(value, str) or value is None: </s> remove         if (not value) and isinstance(value, six.string_types):
 </s> add         if (not value) and isinstance(value, str): </s> remove             return self.delimiter.join(six.text_type(self.base_field.prepare_value(v)) for v in value)
 </s> add             return self.delimiter.join(str(self.base_field.prepare_value(v)) for v in value) </s> remove         if isinstance(value, six.string_types):
 </s> add         if isinstance(value, str): </s> remove         if isinstance(value, six.string_types):
 </s> add         if isinstance(value, str):
        return str(value.title)
      def __init__(self):
          super(TeamField, self).__init__(max_length=100)
  
      def get_db_prep_save(self, value, connection):
        return str(value.title)
  
      def to_python(self, value):
          if isinstance(value, Team):
              return value
          return Team(value)
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. 
tests/serializers/models/base.py