[SPARK-15761][MLLIB][PYSPARK] Load ipython when default python is Python3

## What changes were proposed in this pull request?

I would like to use IPython with Python 3.5. It is annoying when it fails with IPython requires Python 2.7+; please install python2.7 or set PYSPARK_PYTHON when I have a version greater than 2.7

## How was this patch tested
It now works with IPython and Python3

Author: MechCoder <mks542@nyu.edu>

Closes #13503 from MechCoder/spark-15761.