[RFC] Ending Python 2 support

[hcho3]

Given that Python 2 will reach its end of life at the end of this year, we should drop Python 2 support. That is, contributors would no longer be required to write Python code that's compatible with Python 2.7. Also, Python distributions of XGBoost on PyPI will be Python 3 exclusive.

A good time to make this change would be release 0.90.

[hcho3]

@CodingCat Does the Python tracker in XGBoost4J-Spark require Python 2.x?

[trivialfis]

I want to bring this up long time ago. 😄 https://pythonclock.org/

[CodingCat]

@hcho3 I thought it was only tested with python 2, but we can double check travis’s environment to confirm

[hcho3]

Opened vote thread in #4379