pytype is grumpy about the dict.update having a more complex signature than the
one we use here.

pytype error:

  itemregister: Overriding method signature mismatch [signature-mismatch]
    Base signature: 'def builtins.dict.update(self) -> None'.
    Subclass signature: 'def itemregister.update(self, other) -> Any'.
    Parameter 'other' must have a default value.