`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

先從 PR 114574 中引用的配置變更和 Objects/typeobject.c 中的釋放路徑開始，然後檢視關聯的 PR gh-156990。使用透過 PyObject_Malloc 配置的 tp_doc 重現 debug build 擴充功能的情況；當現有擴充功能在兩種受支援的建置模式下都能維持配置與釋放相容時，即表示完成。