http://www.cnblogs.com/freely/p/8022923.html
https://blog.csdn.net/weixin_42874054/article/details/83448531
查看pip版本：
afda@afda-Y720-15IKB:~$ pip --version
pip 19.0.1 from /usr/local/lib/python3.6/dist-packages/pip (python 3.6)python三大神器之virtualenv
原文：https://www.cnblogs.com/2008nmj/p/10340159.html
