python2 python3共存

安装python3、pip3：

sudo yum install epel-release
yum clean all
yum makecache
yum install python36
yum install -y python36-setuptools
easy_install-3.6 pip
pip3 install django

设置python2，python3切换：

注册 python2:

sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 100

注册 python3:

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 150

select python branch:

update-alternatives --config python

设置yum固定为python2：
vim /usr/libexec/urlgrabber-ext-down，修改第一行为：#! /usr/bin/python2.7
vim /usr/bin/yum，修改第一行为：#!/usr/bin/python2.7