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