python3-xlib needed to run clix with Python3

[realslimshanky]

Currently when import error is thrown after running clix for Xlib, installing python-xlib only work when clix is to be run using Python2. In order to make it run for Python3 as well python3-xlib should be installed using OS package manager.

Also, better option would be installing xlib using pip inside the virtual environment. Which will work in a similar way and can be managed gracefully just like any other Python package.

ps: I've tested both the options and they work for Python3.

Whatever would be the option comes out after the result of the discussion, should be reflected on README.md