Often python extension modules define submodules like this:

static PyObject *
initfoo_xpto(void)
{
    PyObject *m;
    m = Py_InitModule3("foo.xpto", foo_xpto_functions, NULL);
    [...]
    return m;
}

PyMODINIT_FUNC
initfoo(void)
{
    PyObject *m;
    PyObject *submodule;
    m = Py_InitModule3("foo", foo_functions, NULL);
    [...]
    submodule = initfoo_xpto();
    Py_INCREF(submodule);
    PyModule_AddObject(m, "xpto", submodule);
}

Unfortunately pydoc does not list these submodules.  Attached patch
fixes it.