[SPARK-25079][PYTHON][BRANCH-2.4] 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 2.4 doesn't like python 3.6...  :(

NOTE: this is just for branch-2.4

PLEASE DO NOT MERGE

Closes apache#24379 from shaneknapp/update-python-executable.

Authored-by: shane knapp <[email protected]>
Signed-off-by: shane knapp <[email protected]>

Author: shaneknapp

python/run-tests.py

@@ -162,7 +162,7 @@ def run_individual_python_test(target_dir, 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")