原来mac下已经安装了python2，如果升级到python3后，想设置默认的python变成python3的方法
vim ~/.bash_profile 再后面追加
alias python="/Library/Frameworks/Python.framework/Versions/3.7/bin/python3.7"
执行以下命令，或者重启系统
source ~/.bash_profile
然后执行
python
Python 3.7.0 (v3.7.0:1bf9cc5093, Jun 26 2018, 23:26:24)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
