> In case this logic is flawed, we know that when remove_subclass() is
> called, exactly one child is removed.  Whether it is us, or some
> previous class, is irrelevant.

remove_subclass() is called when __bases__ is assigned to, so it is
not irrelevant:

>>> class A: pass
... 
>>> class B(A): pass
... 
>>> class C: pass
... 
>>> A.__subclasses__()
[<class '__main__.B'>]
>>> B.__bases__ = C,
>>> A.__subclasses__()
[]
>>> C.__subclasses__()
[<class '__main__.B'>]