Can not install pythonz with only Python3

[y0405]

If Python3 is installed and '/usr/bin/python' does not exist (=Python2 is not installed), pythonz install script can not find python3. So, I can not install pythonz.

pythonz requires Python 2.6, 2.7 or higher

Adding 'command -v python3' or/and '/usr/bin/python3' to PYTHONS variable on pythonz-install script is one of the solution for this problem.

[saghul]

Hey, thanks for looking into it! Mind sending a patch?

[tovmeod]

https://github.com/saghul/pythonz/pull/137