externally unreachable objects with finalizers, and externally unreachable
objects without finalizers reachable from such objects.  This allows us
to call has_finalizer() at most once per object, and so limit the pain of
nasty getattr hooks.  This fixes the failing "boom 2" example Jeremy
posted (a non-printing variant of which is now part of test_gc), via never
triggering the nasty part of its __getattr__ method.