I've stumbled upon further cases of this problem, and a possible serious bug in python 3.
the _PyIOBase_finalize() will invoke a "close" method on the object if the object isn't closed.  This is the same as the object having a __del__ method.  Yet, these objects are not treated as having finalizers by the garbage collector, and indeed, _PyIOBase_finalize() is called by the tp_clear() slot of several derived types. Surely, if these objects define non-trivial 'close' members, they must not be called during garbage collection.