I see the same in Python 3.10 on windows 10.

If I change the relative imports to absolute imports in a couple of functions in multiprocessing.context as below, the attached (pool_error_on_3.9.py) script not longer raises the exception.

    def SimpleQueue(self):
        '''Returns a queue object'''
        from multiprocessing.queues import SimpleQueue
        return SimpleQueue(ctx=self.get_context())

    def Pool(self, processes=None, initializer=None, initargs=(),
             maxtasksperchild=None):
        '''Returns a process pool object'''
        from multiprocessing.pool import Pool
        return Pool(processes, initializer, initargs, maxtasksperchild,
                    context=self.get_context())