mac brew python3.6替换python3.7

第一步：

$ brew unlink python

第二步：安装python3.6

$ brew install --ignore-dependencies https://raw.githubusercontent.com/Homebrew/homebrew-core/f2a764ef944b1080be64bd88dca9a1d80130c558/Formula/python.rb

第三步：切换python3.7跟python3.6

$ brew switch python 3.7.0

$ brew switch python 3.6.5_1