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Hmm, making PyList_* an abstract API doesn't make sense to me. These functions do exactly what they say: they operate on concrete instances of list (they are documented as part of the concrete API). With that reasoning, we should have fallback paths in every function in the concrete APIs; that's a lot of complication added to these C files.
IMO we "should" (or, rather, could) instead add abstract PySequence_Append(), etc. functions if we deem it necessary (just as we already have PyMapping_Keys(), etc.).
By the way, PyList_SetItem() already has an abstract counterpart called PyObject_SetItem(), so changing this one seems useless.