[LIVY-1024] Upgrade Livy to Python3

[ArnavBalyan]

What changes were proposed in this pull request?

• Python2 was deprecated a while back, most systems/environments don't support it anymore
• Bump Livy to Python3, remove Python2 support since Spark 2 has also now been deprecated
• Closes LIVY-1024

How was this patch tested?

• CI / Locally tested the python3 files manully

[ArnavBalyan]

cc @lmccay @gyogal could you please review thanks!

[gyogal]

Thank you, I think your change looks great! There is one more reference to Python 2 that could be changed:
https://github.com/apache/incubator-livy/blob/master/repl/src/main/scala/org/apache/livy/repl/PythonInterpreter.scala#L52
Spark uses "python3" by default:
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/deploy/PythonRunner.scala#L51

[ArnavBalyan]

Thank you, I think your change looks great! There is one more reference to Python 2 that could be changed: https://github.com/apache/incubator-livy/blob/master/repl/src/main/scala/org/apache/livy/repl/PythonInterpreter.scala#L52 Spark uses "python3" by default: https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/deploy/PythonRunner.scala#L51

Updated thanks

[gyogal]

LGTM, thanks, @ArnavBalyan !