brew安装Python3时出现的问题：
Warning: python 3.6.4_4 is already installed, it's just not linked.
You can use `brew link python` to link this version.
发现/usr/local/下没有路径/usr/local/Frameworks
需要新建该路径，并修改权限
解决：
$ sudo mkdir /usr/local/Frameworks
$ sudo chown $(whoami):admin /usr/local/Frameworks
成功：
$ brew link python
Linking /usr/local/Cellar/python3/3.6.4... 1 symlinks created
