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