[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.

bin/pyspark

@@ -50,9 +50,11 @@ if [[ -z "$PYSPARK_DRIVER_PYTHON" ]]; then
   PYSPARK_DRIVER_PYTHON="${PYSPARK_PYTHON:-"$DEFAULT_PYTHON"}"
 fi
 
+WORKS_WITH_IPYTHON=$($DEFAULT_PYTHON -c 'import sys; print(sys.version_info >= (2, 7, 0))')
+
 # Determine the Python executable to use for the executors:
 if [[ -z "$PYSPARK_PYTHON" ]]; then
-  if [[ $PYSPARK_DRIVER_PYTHON == *ipython* && $DEFAULT_PYTHON != "python2.7" ]]; then
+  if [[ $PYSPARK_DRIVER_PYTHON == *ipython* && ! WORKS_WITH_IPYTHON ]]; then
     echo "IPython requires Python 2.7+; please install python2.7 or set PYSPARK_PYTHON" 1>&2
     exit 1
   else