Message 361867 - Python tracker

https://github.com/python/cpython/blob/95905ce0f41fd42eb1ef60ddb83f057401c3d52f/Include/cpython/objimpl.h#L89

_Py_AS_GC and all APIs to get PyGC_Head from PyObject* are not only unstable, but also private.

I thought PyGC_Head must not be used directly from extensions, so we changed its layout.

Since Julien said "my code does not access PyGC_Head at all", I still think it was not real ABI breakage.

I suppose other serious bug (e.g. dungling pointer, missing refcount increment, etc) caused the segfault.