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

先閱讀 PyUnstable_Object_GC_NewWithExtraData() 的 C API 文件，以及 issue 中關於 PyVarObject、free-threaded 建置和穩定 ABI 的討論。當此函式被提升為穩定 API，且擴充功能可以在 abi3t 中將其用於可變大小物件時，工作就完成了。