Inheritance

When breaking a class hierarchy like this, the individual "partial" classes, we call "mixins", will "see" only what is declared directly on them, and on their base-classes. In your example, when writing class A, it does not know anything about class B - you as the author, can know that methods from class B will be present, because methods from class A will only be called from class C, that inherits both.

Your programming tools, the IDE including, can't know that. (That you should know better than your programming aid, is a side track). It would work, if run, but this is a poor design.

If all methods are to be present directly on a single instance of your final class, all of them have to be "present" in a super-class for them all - you can even write independent subclasses in different files, and then a single subclass that will inherit all of them:

from abc import abstractmethod,  ABC

class Base(ABC):

    @abstractmethod
    def method_A_1(self):
        pass

    @abstractmethod
    def method_A_2(self):
        pass

    @abstractmethod
    def method_B_1(self):
        pass

class A(Base):
    def __init__(self, *args, **kwargs):
        # pop consumed named parameters from "kwargs"
        ...
        super().__init__(*args, **kwargs)
        # This call ensures all __init__ in bases are called
        # because Python linearize the base classes on multiple inheritance

    def method_A_1(self):
        ...

    def method_A_2(self):
        ...


class B(Base):
    def __init__(self, *args, **kwargs):
        # pop consumed named parameters from "kwargs"
        ...
        super().__init__(*args, **kwargs)
        # This call ensures all __init__ in bases are called
        # because Python linearize the base classes on multiple inheritance

    def method_B_1(self):
        ...

    ...


class C(A, B):
    pass

(The "ABC" and "abstractmethod" are a bit of sugar - they will work, but this design would work without any of that - thought their presence help whoever is looking at your code to figure out what is going on, and will raise an earlier runtime error if you per mistake create an instance of one of the incomplete base classes)

Composite

This works, but if your methods are actually for wildly different domains, instead of multiple inheritance, you should try using the "composite design pattern".

No need for multiple inheritance if it does not arise naturally.

In this case, you instantiate objects of the classes that drive the different domains on the __init__ of the shell class, and pass its own instance to those child, which will keep a reference to it (in a self.parent attribute, for example). Chances are your IDE still won't know what you are talking about, but you will have a saner design.

class Parent:
    def __init__(self):
        self.a_domain = A(self)
        self.b_domain = B(self)

class A:
    def __init__(self, parent):
        self.parent = parent
        # no need to call any "super...init", this is called
        # as part of the initialization of the parent class

    def method_A_1(self):
        ...

    def method_A_2(self):
        ...


class B:
    def __init__(self, parent):
        self.parent = parent

    def method_B_1(self):
        # need result from 'A' domain:
        a_value = self.parent.a_domain.method_A_1()
        ...

This example uses the basic of the language features, but if you decide to go for it in a complex application, you can sophisticate it - there are interface patterns, that could allow you to swap the classes used for different domains, in specialized subclasses, and so on. But typically the pattern above is what you would need.