[MRG] Python 3 support

[ndawe]

It's coming soon...

TODO:

• Fix failing tests
• Add ROOT + Python 3 builds on Travis

Notes:

• Cython 0.22 is buggy. The generated C++ does not compile due to the incorrect handling of a vector<bool> bit reference.
• ROOT 5 versions v5-34-19 and above have issues with Python 3 because of a new import hook in PyROOT: https://sft.its.cern.ch/jira/browse/ROOT-7108

[ndawe]

Here we go. Python 2 and 3 compatibility and the Travis tests are green. @piti118 @kratsg any comments on this before we merge?

[kratsg]

@ndawe -- Few comments:

• six.py is what you use to have the cython working in python 2 and python 3? I thought it was for python projects.. not cython projects.
• We need to support ROOT=5-34-25 ; this is the final root version being used before Run 2 commences. I believe Root5 is now closed for support and bugs there will not be fixed anymore. Root6 is being fully developed.
• explain the switch between basestring and string_types ? In particular, what's going on here that you changed the type outright: https://github.com/rootpy/root_numpy/pull/177/files#diff-06942ee9ba64664fb84fa1b8cd8ce667L87 [looking at PY2 versus PY3, those are major differences that it seems confusing]

I'm mostly more worried about the ROOT versions. There have been a lot of huge bug fixes related to PyROOT in ROOT 5.34/25 that help with the newer AOD files that people will eventually be using (if they're not using ROOT6).

[ndawe]

Just quickly:

• six.py is used on the python side of things here.
• We still support ROOT=5-34-25 of course. That still works with python 2.X. It just seems that beyond v5-34-19 ROOT is not compatible with python 3.X due to this new import hook in PyROOT. So we can't handle the build matrix as easily with tags above v5-34-19 since they should only be used with python 2.X.
• string_types is the six.py way of being python 2 and 3 compatible since basestring is no longer available in python 3. In python 2 bytes is just an alias for str, but it is a different thing in python 3. Cython handles the python 2 and 3 compatibility here with unicode combined with the Cython options c_string_type=str, c_string_encoding=ascii.

[kratsg]

🔫 all good then!