尝试使用pip install pytest失败，因为pip包有问题6.这里有一个更详细的转储：
Found existing installation: six 1.4.1
DEPRECATION: Uninstalling a distutils installed project (six) has been deprecated and will be removed in a future version.
这是因为卸载distutils项目只会部分卸载项目。
卸载6-1.4.1： 例外： ...
OSError: [Errno 1] Operation not permitted: '/var/folders/gl/6qz_qqhn52g658r712r1rlnh0000gn/T/pip-XD54Hn-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/six-1.4.1-py2.7.egg-info'