> 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.

By leaf, you mean the derived subtype? That sounds a bit quirky (you
need the aforementioned "if"), how about the base heap type instead?

Speaking of which, what does this refactoring bring exactly? The type
declarations are slightly less verbose, but other than that, is there a
point? (using the stable ABI doesn't seem to provide any benefit for
standard extension modules; besides, using PyType_FromSpec is only a
small part of complying with the stable ABI)