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From 15.10 onwards python2 is no longer the default python and is no longer
installed by default.  Switch to python3 which is.  This delta is much
larger than the nominal changes as inconsistent spacing is now an error.
I have also corrected the use of loose strings as comments.

BugLink: http://bugs.launchpad.net/bugs/1506521

Signed-off-by: Andy Whitcroft <apw@canonical.com>