Get mypy to accept subtype of generic type as a method argument

I'm trying to extract a pattern that we are using in our code base into a more generic, reusable construct. However, I can't seem to get the generic type annotations to work with mypy.

Here's what I got:

from abc import (
    ABC,
    abstractmethod
)
import asyncio
import contextlib
from typing import (
    Any,
    Iterator,
    Generic,
    TypeVar
)

_TMsg = TypeVar('_TMsg')

class MsgQueueExposer(ABC, Generic[_TMsg]):

    @abstractmethod
    def subscribe(self, subscriber: 'MsgQueueSubscriber[_TMsg]') -> None:
        raise NotImplementedError("Must be implemented by subclasses")

    @abstractmethod
    def unsubscribe(self, subscriber: 'MsgQueueSubscriber[_TMsg]') -> None:
        raise NotImplementedError("Must be implemented by subclasses")


class MsgQueueSubscriber(Generic[_TMsg]):

    @contextlib.contextmanager
    def subscribe(
            self,
            msg_queue_exposer: MsgQueueExposer[_TMsg]) -> Iterator[None]:
        msg_queue_exposer.subscribe(self)
        try:
            yield
        finally:
            msg_queue_exposer.unsubscribe(self)


class DemoMsgQueSubscriber(MsgQueueSubscriber[int]):
    pass

class DemoMsgQueueExposer(MsgQueueExposer[int]):

    # The following works for mypy:

    # def subscribe(self, subscriber: MsgQueueSubscriber[int]) -> None:
    #     pass

    # def unsubscribe(self, subscriber: MsgQueueSubscriber[int]) -> None:
    #     pass

    # This doesn't work but I want it to work :)

    def subscribe(self, subscriber: DemoMsgQueSubscriber) -> None:
        pass

    def unsubscribe(self, subscriber: DemoMsgQueSubscriber) -> None:
        pass

I commented out some code that works but doesn't quite fulfill my needs. Basically I want that the DemoMsgQueueExposer accepts a DemoMsgQueSubscriber in its subscribe and unsubscribe methods. The code type checks just fine if I use MsgQueueSubscriber[int] as a type but I want it to accept subtypes of that.

I keep running into the following error.

generic_msg_queue.py:55: error: Argument 1 of "subscribe" incompatible with supertype "MsgQueueExposer"

I feel that this has something to do with co-/contravariants but I tried several things before I gave up and came here.

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