We only support Puppet versions available for platforms up through
Ubuntu Xenial, which provides Python 3.5 as its python3 interpreter.
Recently pip, and by extension get-pip.py, ceased supporting Python
<3.6 but added stable URLs for the last releases to support earlier
interpreters. Switch to the URL for a get-pip.py which will still
work for Xenial servers.
Also fix a couple of new linter failures.
Change-Id: I042bfc330e4d562313cf0dbf2be7d0e2d5779f40