_PyCode_SetExtra() uses two memory block for code extras. By changing how memory is accessed and allocated, it would be possible to use a single memory block. Was it on purpose to use two memory blocks?

See for example PyTupleObject which uses a single memory block vs PyListObject which uses two memory blocks.

typedef struct {
    PyObject_VAR_HEAD
    PyObject *ob_item[1];

    /* ob_item contains space for 'ob_size' elements.
     * Items must normally not be NULL, except during construction when
     * the tuple is not yet visible outside the function that builds it.
     */
} PyTupleObject;