解决flask中app引用不到的问题

大型项目建议将app单独提出来定义

参考
https://github.com/xdqt/python-postgresql-flask-graphql

https://stackoverflow.com/questions/14415500/common-folder-file-structure-in-flask-app

https://flask.palletsprojects.com/en/2.0.x/patterns/packages/