PyCCC: single-source Python 2/3 support

[avirshup]

Story: PyCCC supports python 2 and 3 with a single source branch.

We'll also be porting nbmolviz and moldesign, so we need to document:

• migration process (starting with Modernize and six.py
• dev environment setup for simultaneous 2/3
• testing simultaneous 2/3
• code compatibility guidelines

[avirshup]

WIP: Decided to use the future package. Current work is in py3_future branch. Current working assumption is that we can write mostly-clean python 3 code if we include this at the top of each file:

from __future__ import print_function, unicode_literals, absolute_import, division
from future import standard_library
standard_library.install_aliases()
from future.builtins import *

String handling, however, will require much more care than with pure python 2. This branch includes a substantial file handling refactor to address py3's new str and bytes types. The various job running APIs (subprocess, CCC, docker) will also need more explicit codecs.

[avirshup]

Current status: can now explicitly request an interpreter when starting a python job. Tests are running successfully in both languages, including calls from python 2 to 3 and vice versa.

Still to migrate / test:

• CCC interface
• Job status widet

[avirshup]

Closing - unlikely to happen on our end in the near future.