单例模式（python）
python 的单例模式需要重写__new__()和 __init__()
需要注意，标识符__和_区别
参考资料：
https://www.cnblogs.com/huchong/p/8244279.html
https://www.cnblogs.com/tangkaishou/p/9246353.html
https://blog.csdn.net/qq_43439853/article/details/99584193
https://cloud.tencent.com/developer/article/1582867
https://docs.python.org/zh-cn/3/tutorial/classes.html#private-variables
