def _generate_typevars(self):
        # type: () -> None
        """
        Creates type variables that are used by the type signatures for
        _process_custom_annotations.
        """
        self.emit("T = TypeVar('T', bound=bb.AnnotationType)")
        self.emit("U = TypeVar('U')")
        self.import_tracker._register_typing_import('TypeVar')
        self.emit()
Creates type variables that are used by the type signatures for
        _process_custom_annotations.