It seems that PyOS_AfterFork_Child() needs to do something like

PyThreadState *tstate = PyThreadState_Get();
PyObject *wr = _PyObject_CAST(tstate->on_delete_data);
PyObject *obj = PyWeakref_GET_OBJECT(wr);
lockobject *lock;
if (obj != Py_None) {
    lock = (lockobject *) obj;
    if (lock->locked) {
        /* Leak memory on purpose. */
        lock->locked = 0;
    }
}

before the call to _PyEval_ReInitThreads.

Maybe encapsulate that as PyThread_ReInitThreads().

The locks in the threads in the child need to be cleared before _PyThreadState_DeleteExcept() so that Python does not try to release the locks in the child.