Expose Py_SIZE, Py_IS_TYPE, Py_SET_SIZE as functions in the Stable ABI

[encukou]

Feature or enhancement

Py_SIZE & Py_SET_SIZE have issues -- I wish ob_size was much less public than it is -- but they can be used correctly and don't currently have good alternatives for some use cases.
Also, they're available as static inline functions when Py_LIMITED_API is defined, so many people treat them as part of Limited API already. The same goes for Py_IS_TYPE.

Let's turn them into real function symbols, and keep the static inline variants in the non-Limited API only.

---

Note that I don't propose exposing Py_SET_TYPE this way. The proper way to change an object's type is setting the __class__ Python attribute, which does proper checks and reference counting.

Linked PRs

• gh-139165: Make Py_SIZE, Py_IS_TYPE,Py_ SET_SIZE regular functions in stable ABI #139166