>>> Since tp_visit is GC specific thing, I think it wold be more convenient make a special case for object types.

I don't think that follows: The gc defers the logic of what and what should not be visited to the tp_traverse implementation of every object. The GC must be agnostic of the types it receives and heap types here are not special.

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Also, if we make an exception in the GC for this special case, all tp_traverse of all those functions will be incorrect. Someone reading those tp_traverse can say "Oh, why these functions do not visit the type if they own s reference to it, this looks incorrect" and then we need to explain that that is an exception in the GC just because we were lazy to implement them correctly.