Avoid double checking `tp_as_number`, `tp_as_sequence` and `tp_as_mapping`

Feature or enhancement

The PyTypeObject fields tp_as_number, tp_as_sequence and tp_as_mapping can be set to NULL
which means that any code that wants to access a field in those structs needs to first check for NULL.
e.g. checking if an object is a sequence:
if (Py_TYPE(s)->tp_as_mapping && Py_TYPE(s)->tp_as_mapping->mp_length) {

this is ugly, a bit error prone and slower than necessary.
Instead, during PyType_Ready, any of these fields that are NULL should be made to pointer to a struct full of NULLs.
This would remove the need for the extra check:
if (Py_TYPE(s)->tp_as_mapping->mp_length) {
since we would be guaranteed that Py_TYPE(s)->tp_as_mapping is never NULL.

Immutable types can all share common constant structs, so this should use very little extra memory.

Linked PRs

• gh-149317
• gh-149476

Commencez par PyType_Ready et suivez le traitement de tp_as_number, tp_as_sequence et tp_as_mapping, puis examinez les schémas d’accès décrits dans l’issue. Le travail est terminé lorsque les slots NULL sont représentés de manière sûre par des structures partagées dont tous les éléments sont NULL, sans les vérifications supplémentaires, et que les tests CPython pertinents passent.