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Antoine: Py_DECREF calls tp_dealloc directly, so the type needs to be DECREFed in the course of tp_dealloc. I don't think there is any alternative to that.
One may wonder why regular extension types don't do that: this is because of a hack that excludes static (non-heap) types from being reference counted in their instances. Heap types do refcount their types, consequently, subtype_dealloc also DECREFs the type.
I certainly agree that this is muddy, in particular when it comes to subtyping where the derived subtype calls the base tp_dealloc. In an ideal world, object_dealloc would decref the type, and subtypes would be required to call the base type's dealloc. However, I feel that this cannot be changed before Python 4.
So I'd propose that it is actually the leaf subtype which decrefs ob_type. The check whether you are the leaf type is then done by checking whether tp_dealloc is the one you are "in" right now.