[SPARK-25079][PYTHON][BRANCH-2.3] update python3 executable to 3.6.x

## What changes were proposed in this pull request?

have jenkins test against python3.6 (instead of 3.4).

## How was this patch tested?

extensive testing on both the centos and ubuntu jenkins workers revealed that 2.3 probably doesn't like python 3.6... :(

NOTE: this is just for branch-2.3

PLEASE DO NOT MERGE

Author: shane knapp <incomplete@gmail.com>

Closes #24380 from shaneknapp/update-python-executable-2.3.

Author: shaneknapp

python/run-tests.py

@@ -114,7 +114,7 @@ def run_individual_python_test(test_name, pyspark_python):
 
 
 def get_default_python_executables():
-    python_execs = [x for x in ["python2.7", "python3.4", "pypy"] if which(x)]
+    python_execs = [x for x in ["python2.7", "python3.6", "pypy"] if which(x)]
     if "python2.7" not in python_execs:
         LOGGER.warning("Not testing against `python2.7` because it could not be found; falling"
                        " back to `python` instead")