网络不稳定在安装时 会出现
ERROR: Could not find a version that satisfies the requirement virtualenv (from versions: none)
ERROR: No matching distribution found for virtualenv
因此再安装模块时指定国内镜像，比如安装虚拟环境时
pip install virtualenv -i http://pypi.douban.com/simple/ --trusted-host pypi.douban.com