Added C API for extending types whose instance memory layout is opaque:
:c:member:`PyType_Spec.basicsize` can now be zero or negative,
:c:func:`PyObject_GetTypeData` can be used to get subclass-specific data,
and :c:macro:`Py_TPFLAGS_ITEMS_AT_END` can be used to safely extend
variable-size objects. See :pep:`697` for details.