certbot has problem setting up the virtual environment解决办法

　出现这个问题，基本上是由于apt-get源在国外导致的，我们可以手动替换成国内的源

1. 编辑源列表文件

sudo vim /etc/apt/sources.list

2. 将原来的列表删除，添加如下内容（中科大镜像源）

deb http://mirrors.ustc.edu.cn/ubuntu/ xenial main restricted universe multiverse 
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-security main restricted universe multiverse  
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse  
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-proposed main restricted universe multiverse  
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse  
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial main restricted universe multiverse  
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial-security main restricted universe multiverse  
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse  
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial-proposed main restricted universe multiverse  
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse

3. 最后重新运行

sudo apt-get update