From Python-3.3 source code, Python/pystate.c. Note the comment: I have no recollection of requesting this feature, but it must have been important at the time ;-).

pystate.c

/* Routines for advanced debuggers, requested by David Beazley.
   Don't use unless you know what you are doing! */

PyInterpreterState *
PyInterpreterState_Head(void)
{
    return interp_head;
}

PyInterpreterState *
PyInterpreterState_Next(PyInterpreterState *interp) {
    return interp->next;
}

PyThreadState *
PyInterpreterState_ThreadHead(PyInterpreterState *interp) {
    return interp->tstate_head;
}

PyThreadState *
PyThreadState_Next(PyThreadState *tstate) {
    return tstate->next;
}

@dabeaz can you explain this dark magic ?:))