In the following situation:
```python
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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