ABCs, even though they are almost always at the bottom of an inheritance hierarchy, should still do the right thing in the face of being in the middle of an MRO. That means that they should call super() as appropriate. So for methods that return a value, blindly call super(). For methods that do not necessarily return anything (e.g. invalidate_caches()), check if super() as the method and it is callable and if that is true then make the super() call.

This is not backwards-compatible as it is new semantics people will rely on, but neither is it a bug but a bad design decision on my part.