参考：
在升级 six 时遇到无法安装的问题，错误日志如下：
Cannot uninstall 'six'. It is a distutils installed project and thus we cannot accurately determine which files belong to it which would lead to only a partial uninstall.
导致错误的原因：Six issue when installing package
解决方法：
sudo pip install six --upgrade --ignore-installed six
2018.8