Matthew Pickering authored Dec 23, 2016 and Ben Gamari committed Dec 23, 2016

We now require python3 for the testsuite so rather than require two
versions of python it makes sense to use python3 for the linters as
well.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2888