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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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()