Py_buffer.shape requires a Py_ssize_t* pointer. It's not convenient. For example, the array module uses:

static int
array_buffer_getbuf(arrayobject *self, Py_buffer *view, int flags)
{
    ...
    if ((flags & PyBUF_ND)==PyBUF_ND) {
        view->shape = &((PyVarObject*)self)->ob_size;
    }
    ...
    return 0;
}

This code is not compatible with a fully opaque PyObject structure:
https://bugs.python.org/issue39573#msg401395