cliff is being developed under Python 3.
cliff should be installed into the same site-packages area where the application and extensions are installed (either a virtualenv or the global site-packages). You may need administrative privileges to do that. The easiest way to install it is using pip. For example:
$ pip install cliff
The source is hosted on OpenDev: https://opendev.org/openstack/cliff
Please report bugs through the storyboard: https://storyboard.openstack.org/#!/project/openstack/cliff