Basic Python 3 support for testsuite driver (Trac #9184)

Summary:
Most of the changes is adaptation of old Python 2 only code.
My priority was not breaking Python 2, and so I avoided bigger
changes to the driver. In particular, under Python 3 the output
is a str and buffering cannot be disabled.

To test, define PYTHON=python3 in testsuite/mk/boilerplate.mk.

Thanks to aspidites <emarshall85@gmail.com> who provided the initial patch.

Test Plan: validate under 2 and 3

Reviewers: hvr, simonmar, thomie, austin

Reviewed By: thomie, austin

Subscribers: aspidites, thomie, simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D233

GHC Trac Issues: #9184