pip error: AttributeError: '_NamespacePath' object has no attribute 'sort'

I encountered the error of pip when I tried to install tensor2tensor.

AttributeError: '_NamespacePath' object has no attribute 'sort'

I used python3.6.0 and pip 8.1 at first. I thought it was because the versions are too old so I upgraded my python version to 3.6.3 and pip to 18.1. However, the error still existed.
Then, I searched online and found this github issue:

https://github.com/pypa/setuptools/issues/885

One of the solutions worked! I uninstalled my setuptools and then reinstalled the newest version.
The commands are very easy.

pip uninstall setuptools
pip install setuptools

That's it!