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Here's another idea: have a new type field (a tp_something) that exposes the GC priority for this type. It could be an integer between -8 and 7, for example (0 being the default). Then tp_finalize would be called in sorted priority order.
The tricky part of course is to make this acceptable performance-wise (i.e. avoid walking the collectable list 16 times, once for each priority level). For example by creating bucket lists for each priority level (only for objects with a non-NULL tp_finalize).