`tp_doc` switch from `PyObject_Malloc` to `PyMem_Malloc` is not backwards compatible

Bug report

In https://github.com/python/cpython/pull/114574 we switched a number of non-PyObject allocations from PyObject_Malloc to PyMem_Malloc, including tp_doc on PyHeapTypeObjects.

Unfortunately, this isn't backwards compatible because C-API extensions may allocate tp_doc contents, which are then freed by CPython in type_dealloc. For example, pybind11 allocates memory for the docstring using PyObject_MALLOC. This leads to crashes when using pybind11 in debug builds of Python 3.13: the allocation uses PyObject_MALLOC, but the memory is freed using PyMem_Free.

We should consider reverting the change to tp_doc and figure out a way to allocate the doc in a way that's both safe (in the free-threaded build) and doesn't break backwards compatibility (in the default build).

cc @erlend-aasland

Linked PRs

• gh-156990

Beginne mit der in PR 114574 zitierten Allocationsänderung und dem Freigabepfad in Objects/typeobject.c und prüfe anschließend den verknüpften PR gh-156990. Reproduziere den Fall einer Erweiterung im Debug-Build mit einem mittels PyObject_Malloc allozierten tp_doc; abgeschlossen ist die Aufgabe, wenn Allokation und Freigabe für bestehende Erweiterungen in beiden unterstützten Build-Modi kompatibel bleiben.