pip install --upgrade pip 命令行执行异常，read time out，解决方法

命令如下

python -m pip install --upgrade pip -i https://pypi.tuna.tsinghua.edu.cn/simple

显示成功，如下：

[root@localhost ~]# python3 -m pip install --upgrade pip -i https://pypi.tuna.tsinghua.edu.cn/simple
Looking in indexes: https://pypi.tuna.tsinghua.edu.cn/simple
Collecting pip
  Downloading https://pypi.tuna.tsinghua.edu.cn/packages/54/2e/df11ea7e23e7e761d484ed3740285a34e38548cf2bad2bed3dd5768ec8b9/pip-20.1-py2.py3-none-any.whl (1.5MB)
     |████████████████████████████████| 1.5MB 636kB/s 
Installing collected packages: pip
  Found existing installation: pip 19.3.1
    Uninstalling pip-19.3.1:
      Successfully uninstalled pip-19.3.1
Successfully installed pip-20.1