Forking as a side effect of importing a module is evil.  I think raising a RuntimeError is preferable to trying to make it Just Work.

But maybe one could do

    void
    _PyImport_ReInitLock(void)
    {
        if (import_lock != NULL) {
            import_lock = PyThread_allocate_lock();
            PyThread_acquire_lock(import_lock, WAIT_LOCK);
        }
        import_lock_thread = PyThread_get_thread_ident();
        _PyImport_ReleaseLock();
    }