ubuntu安装 python2.7（参考网址）
https://linuxize.com/post/how-to-install-pip-on-ubuntu-18.04/

1. 首先使用以下命令更新包列表：

$ sudo apt-get update

2. 然后安装python2.7

$ sudo apt-get install python2.7
$ sudo apt update

3. 安装python2.7 pip

$ sudo apt install python-pip
$ pip install --upgrade pip
删除python3.5
https://www.jianshu.com/p/b6959d0c7353