Standardize on Python 3 by default

[brianv0]

Jupyter and other software is ending active support Python 2 (IPython 5.x LTS) support at the end of the year, with all support ceasing in 2019. While it's feasible that some support could still happen, by not encouraging Python 3 by default we risk running into support issues at NERSC and Jupyterlab. Similarly, all integration tests in DM for 2018 and beyond will be performed using python 3 by default.

I'd propose we discourage any new code from being written in python 2, and encourage python 3 compatibility by December 2017 as a collaboration, otherwise we risk losing support for running collaboration software using Jupyter at NERSC.

[esheldon]

be prepared for lots of confusing errors related to strings

[heather999]

Do we have a page that can aid those migrating from Python 2 to Python 3 for existing projects @jchiang87? If not, is there a list of the top 10 most common things that need to be done? I'm happy to make a page.

[jchiang87]

We don't have one for DESC, afaik, but I made some links at the Camera Data Handling page that may be a good start for a DESC page,

https://confluence.slac.stanford.edu/display/LSSTCAM/Python+V3+Migration​

Note that these are all links to the DM documentation.

[drphilmarshall]

This came up a couple of times today: DESCQA are in transition to python 3, and Chris Walter pointed out it would make life much easier supporting people at NERSC.

[heather999]

Py2 is deprecated, thinking it is safe to assume everyone has moved on to Py3