在pip安装allenmlp的时候，出现如下错误：
ERROR: Cannot uninstall 'greenlet'. 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.
原因：distutils安装的包缺少相应的metadata，pip不能完全卸载。
解决方法：distutils没有提供相关的卸载命令，直接找到包的位置，删除相应的文件即可，比如
mv ~/anaconda3/lib/python3.6/site-packages/greenlet* /tmp/
之后再安装即可。
参考
- https://github.com/pypa/pip/issues/5247
- https://github.com/pypa/pip/pull/3389
- https://github.com/blockstack/blockstack-core/issues/504