Draft: gh-139103: Use _PyObject_GetMethodStackRef for slot_tp_new

[eendebakpt]

Use _PyObject_GetMethodStackRef for slot_tp_new to avoid refcount contention in namedtuple.

Marked draft: the free-threading bottleneck is not yet resolved

• Issue: Poor thread scaling when constructing instances or accessing attributes #139103

[colesbury]

@eendebakpt: _PyObject_GetMethodStackRef is not going to be efficient here. It's only specialized if the passed in object's type has a tp->tp_getattro of PyObject_GenericGetAttr. In this case we are getting the __new__ function on a type instance. Typically a type's type is PyType_Type and that has tp_getattro of _Py_type_getattro.