Promote `PyUnstable_Object_GC_NewWithExtraData()` to the stable API

Feature or enhancement

Proposal:

Allocating variable sized Python objects via the C API is a genuinely useful operation. It is e.g. needed to

• implement custom data structures (tuples, lists)
• implement binding libraries (like nanobind) that co-locate binding data structures with Python objects

So far, the standard way to create them was via PyVarObject, and this was supported even in stable ABI builds. Unfortunately, this data structure becomes inaccessible with the upcoming free-threaded flavor of the stable ABI.

I would like to propose that the function PyUnstable_Object_GC_NewWithExtraData() be promoted to the stable API to provide an escape hatch for extensions that require variable sized objects in abi3t. This will enable extensions to implement variable sized objects without depending on the specifics of PyVarObject.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-148499

Bắt đầu bằng cách đọc tài liệu C API cho PyUnstable_Object_GC_NewWithExtraData() và phần thảo luận của issue về PyVarObject, các bản build free-threaded và stable ABI. Công việc hoàn tất khi hàm này được đưa vào stable API và các extension có thể sử dụng nó cho các đối tượng có kích thước thay đổi trong abi3t.