Creates the form element.

def _create_plain_field(self, attr, options):
        """Creates the form element."""
        method = self._get_field_method(attr.py_type) or self._create_other_field
        klass, options = method(attr, options)

        if attr.is_unique:
            options['validators'].append(validators.UniqueEntityValidator(attr.entity))

        return klass, options