Reflect in setup classifiers that support for python <3 was dropped

[maapxa]

Support for python < 3 was dropped in this commit. Would be nice to have that changed in the classifiers section, and test requirements, in setup.py

[felipeek]

I'm getting an error trying to install ipykernel from pip and I think this is related to this issue.

Collecting ipykernel>=4.5.1 (from ipywidgets==7.1.0->halo==0.0.14->infraboxcli)
  Using cached https://files.pythonhosted.org/packages/e1/cb/0129f461e8a4823fe662df4992d6ce8a0b593a5e86c6de78d4b882a0cf44/ipykernel-5.0.0.tar.gz
    Complete output from command python setup.py egg_info:
    ERROR: ipykernel requires Python version 3.4 or above.

This happens because https://github.com/ipython/ipykernel/blob/master/setup.py has
#!/usr/bin/env python

Python 3 is called using python3, not python