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

首先阅读 PyUnstable_Object_GC_NewWithExtraData() 的 C API 文档，以及 issue 中关于 PyVarObject、free-threaded 构建和稳定 ABI 的讨论。当此函数被提升为稳定 API，并且扩展可以在 abi3t 中将其用于可变大小对象时，工作就完成了。