fix: Tune mixin types, so linters could recognize them better

[mykola-mokhnach]

This should improve auto-complete suggestions in the IDE and address type warnings in PyCharm.

It would be necessary to use protocols if we want to give mypy a hint about access to other mixin classes. For example:

class CanValidateCoordinates(Protocol):
    def validate(x, y): ...

......

Here we have some other mixin class, which implements the `CanValidateCoordinates` protocol
......
T = TypeVar('T', bound=Union['MixinClass', CanValidateCoordinates])
....
def mixin_method(self: T):
   self.validate(x, y)  # <--- mypy appreciates

[KazuCocoa]

👀

[ki4070ma]

[Question] Is this for Pycharm?

[mykola-mokhnach]

exactly. Pycharm has its own linter, which shows warnings