Travis CI: Fix the remaining Python 3 issues

[cclauss]

Checklist

• npm install && npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

Resolve issues related to the following items being removed from Python 3:

• basestring
• cmp()
• long
• reload() (moved, not removed)
• sys.setdefaultencoding()
• unicode

[rvagg]

sweet, @nodejs/python can we get at least one other python person to +1 this please? then we can get it shipped.

[bnoordhuis]

LGTM, thanks!

[rvagg]

landed in master

@cclauss just for educational purposes so you can start landing these yourself: I've rewritten both commits to prepend a "subsystem" like we do in nodejs/node. The first commit is "gyp: ..." and the second is "test: ...". These prefixes make the changelog a little more digestable and our changelog-maker tool can group them. I've also added the following metadata to the commits, again for our tools (changelog-maker and branch-diff):

PR-URL: https://github.com/nodejs/node-gyp/pull/1793
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>