[SPARK-25079][PYTHON] 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.

NOTE:  this will need to be backported to all active branches.

Closes #24266 from shaneknapp/updating-python3-executable.

Authored-by: shane knapp <incomplete@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

dev/run-tests.py

@@ -546,7 +546,9 @@ def main():
         hadoop_version = os.environ.get("AMPLAB_JENKINS_BUILD_PROFILE", "hadoop2.7")
         test_env = "amplab_jenkins"
         # add path for Python3 in Jenkins if we're calling from a Jenkins machine
-        os.environ["PATH"] = "/home/anaconda/envs/py3k/bin:" + os.environ.get("PATH")
+        # TODO(sknapp):  after all builds are ported to the ubuntu workers, change this to be:
+        # /home/jenkins/anaconda2/envs/py36/bin
+        os.environ["PATH"] = "/home/anaconda/envs/py36/bin:" + os.environ.get("PATH")
     else:
         # else we're running locally and can use local settings
         build_tool = "sbt"

python/run-tests.py

@@ -160,7 +160,7 @@ 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")