macOS，virtualenv是pip install安装的。用homebrew把python 3.6升级成了3.7，然后virtualenv就罢工了，每次都提示：/usr/local/bin/virtualenv: bad interpreter: /usr/local/opt/python3/bin/python3.6: no such file or directory

解决方案，重装virtualenv:

sudo pip uninstall virtualenv
sudo pip install virtualenv