Mostly mechanical changes for Python3 support.

Everything should still work fine in Python 2. With this change (and
the others I've just landed), Python 3 should work on at least
Linux, but I need to do more sanity-checking on Mac and Win.

The changes in this CL are all mechanical -- things like
print() instead of print, `key in dict` rather than dict.has_key(key),
and switching to use .decode('utf8') to handle the binary streams
returned from a subprocess (and a few other things).

Most of this work is derived from Ashley.Whetter@gmail.com's
original work in https://codereview.chromium.org/1454433002/.

Bug: gyp:36

Change-Id: Ie04ebcf2d82e7b8ff34c6a112215eac46af688ba
Reviewed-on: https://chromium-review.googlesource.com/c/1357805


Reviewed-by: Mark Mentovai <mark@chromium.org>