Mac环境pip install报错，如下：
The directory '/Users/xxx/Library/Caches/pip/http' or its parent directory is not owned by the current user and the cache has been disabled. Please check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.
OSError: [Errno 1] Operation not permitted
解决方案：--user
pip install nltk --user -U
