Super Python (part 1)

Honestly, I'm starting to think that this person can get 80% of what they want by turning their mixins into functions like this:

T = typing.TypeVar('T', bound=SomeBaseClass)

def Mixin(Parent: type[T]) -> type[T]:
    class MixinClass(Parent):
        # Put mixin code here.
    return MixinClass

# Elsewhere in the codebase:

class SomeClass(SomeBaseClass):
    # Put the parent's code here.

class Derived(Mixin(SomeClass)):
    # Put the child's code here.

You don't need to futz with metaclasses to do that, you get the nice class Derived(Mixin(SomeClass)): syntax that this person seems to want, as well as an explicit, purely linear inheritance hierarchy, and (I think) you can even run this through a type-linter. The only downside is that pickling tends to dislike such dynamic classes, but frankly pickling is evil anyway.

If even that is too much typing, you can make a decorator that automagicks your mixin classes into Mixin()-like functions.