解决 Cannot uninstall 'pyparsing' 问题
在安装 pydot 时遇到依赖 pyparsing 无法更新问题：
ERROR: Cannot uninstall 'pyparsing'. It is a distutils installed project and thus we cannot accurately determine which files belong to it which would lead to only a partial uninstall.
解决方法是手动重装最新版 pyparsing：
sudo pip install -I pyparsing==2.2.0
