Ben Gamari authored Nov 29, 2016 and Ben Gamari committed Nov 29, 2016

Summary:
It turns out that Phyx's fix for #12554 (D2684) still fails with mingw-w64
python 2.7. However, Python 3 (both msys2 and mingw-w64) work fine. Given that
supporting Python 2 has already become rather tiresome (as @thomie warned it
would), let's just move to python3 by default.

Test Plan: Validate

Reviewers: austin, Phyx

Reviewed By: Phyx

Subscribers: Phyx, thomie

Differential Revision: https://phabricator.haskell.org/D2766

GHC Trac Issues: #12554