          # ImageFieldFile for ImageFields; it's also conceivable that user
          # subclasses might also want to subclass the attribute class]. This
          # object understands how to convert a path to a file, and also how to
          # handle None.
          if isinstance(file, six.string_types) or file is None:
              attr = self.field.attr_class(instance, self.field, file)
              instance.__dict__[self.field.name] = attr
  
          # Other types of files may be assigned as well, but they need to have
          # the FieldFile interface added to them. Thus, we wrap any other type of
 
</s> remove             # Class names must be ASCII in Python 2.x, so we forcibly coerce it
            # here to break early if there's a problem.
            to = str(to)
 </s> add  </s> remove             is_candidate = isinstance(obj, (bytes, six.string_types)) or hasattr(obj, '__geo_interface__')
 </s> add             is_candidate = isinstance(obj, (bytes, str)) or hasattr(obj, '__geo_interface__') </s> remove         if isinstance(ds_input, six.string_types) and json_regex.match(ds_input):
 </s> add         if isinstance(ds_input, str) and json_regex.match(ds_input): </s> remove         elif isinstance(unique, six.string_types):
 </s> add         elif isinstance(unique, str): </s> remove         return six.text_type(value)
 </s> add         return str(value)
        if isinstance(file, str) or file is None:
          # ImageFieldFile for ImageFields; it's also conceivable that user
          # subclasses might also want to subclass the attribute class]. This
          # object understands how to convert a path to a file, and also how to
          # handle None.
        if isinstance(file, str) or file is None:
              attr = self.field.attr_class(instance, self.field, file)
              instance.__dict__[self.field.name] = attr
  
          # Other types of files may be assigned as well, but they need to have
          # the FieldFile interface added to them. Thus, we wrap any other type of
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