[Merge] ubuntu-dev-tools:python3 into ubuntu-dev-tools:master

[Mattia Rizzolo]

Notes to self, things to do while/after merging:

* remove "pylint bug: http://www.logilab.org/ticket/46273"
* scottk's commit touches changelog, check gbp dch
* needs a X-Python3-Version >= 3.6
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https://code.launchpad.net/~ubuntu-dev/ubuntu-dev-tools/+git/ubuntu-dev-tools/+merge/372305
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