Mypy complains

mytest.py

import abc
import typing
import dataclasses


from functools import singledispatchmethod


@dataclasses.dataclass
class Root:
    pass


@dataclasses.dataclass
class Child(Root):
    name: str


@dataclasses.dataclass
class Parent(Child):
    family: int


class Interface(metaclass=abc.ABCMeta):
    @abc.abstractmethod
    def handle(self, thing: Root) -> None:
        raise NotImplementedError


class Implementer(Interface):
    @singledispatchmethod
    def handle(self, thing: Root) -> None:
        raise ValueError

    @singledispatchmethod
    def _handle_child(self, thing: Child) -> None:
        print("ok", thing)

    @singledispatchmethod
    def _handle_parent(self, thing: Parent) -> None:
        print("ok", thing)

output.txt

mypy mytest.py 
mytest.py:32: error: Signature of "handle" incompatible with supertype "Interface"
Found 1 error in 1 file (checked 1 source file)