Bug report

Bug description:

The original problem - msgspec/msgspec#960

In Python 3.13 PyObject_GC_New does not do initialization inline values (aka _PyObject_InitInlineValues).
In Python 3.14 PyObject_GC_New does initialization inline values - https://github.com/python/cpython/blob/main/Python/gc.c#L2377-L2379

Are there any reasons why PyObject_GC_New doesn't do this initialization in Python 3.13?

I tried to fix msgspec by adding a call to the private function _PyObject_InitInlineValues, but this causes other problems.
Can you tell me how to fix this place more correctly? I just came up with the idea to make a copy of the _PyObject_InitInlineValues function in msgspec for Python 3.13.

CPython versions tested on:

3.13

Operating systems tested on:

Linux