[pylint] Include name of base class in message for redefined-slots-in-subclass (W0244)

[dylwil3]

In the following situation:

class Grandparent:
  __slots__ = "a"

class Parent(Grandparent): ...

class Child(Parent):
  __slots__ = "a"

the message for W0244 now specifies that a is overwriting a slot from Grandparent.

To implement this, we introduce a helper function iter_super_classes which does a breadth-first traversal of the superclasses of a given class (as long as they are defined in the same file, due to the usual limitations of the semantic model).

Note: Python does not allow conflicting slots definitions under multiple inheritance. Unless I'm misunderstanding something, I believe It follows that the subposet of superclasses of a given class that redefine a given slot is in fact totally ordered. There is therefore a unique nearest superclass whose slot is being overwritten. So, you know, in case anyone was super worried about that... you can just chill.

This is a followup to #9640 .

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[dhruvmanila]

This looks great, thanks for following up quickly on this!

I just have one question about whether to use depth-first or breath-first traversal otherwise this looks good to go.

[dhruvmanila]

What's the reason for doing breadth-first? I think the original implementation did depth-first traversal.

[dylwil3]

I guess my intuition is that the superclass graph will usually be deeper than it is wide (but maybe that assumption is off), and that the width is often pretty small in any event. Does that seem right to you?

[dylwil3]

To save you the re-review, I'm gonna merge this in. But if you wake up in the middle of the night shouting "depth first!" I will be happy to change it back 😄