pkg_resources.DistributionNotFound: The ‘pip==9.0.0‘ distribution was not found and is required

升级pip后出现问题：

解决方法：

easy_install pip==9.0.0

参考博客：
bug宝典Python篇 pkg_resources.DistributionNotFound: The ‘pip==7.1.0’ distribution was not found and is