centos更新python3
yum update
yum install python3.4 -y
/usr/bin/python* -al
ln -s python3 /usr/bin/python
vi /usr/bin/yum
vi /usr/libexec/urlgrabber-ext-down
python →python2
python -V
python2 -V
yum install python34-setuptools
easy_install-3.4 pip
pip3 install flask
先查看已安装的版本
yum list installed | grep python