update script and doc to use stdeb 0.7.1

I think this is the way to get it working with the current packages available. It also removes some of the code blocks which differ between Python 2 and 3.

For `catkin_pkg` it generates the "right" Python 2 version dependency for me:

    Build-Depends: python-all (>= 2.6.6-3), debhelper (>= 7.4.3)
    X-Python-Version: >= 2.6

@tfoote @wjwwood Please review - eventually trying it in a virtualenv (to not break your working system using stdeb 0.6) might be a good idea.