[SPARK-2470] PEP8 fixes to PySpark

This pull request aims to resolve all outstanding PEP8 violations in PySpark.
I'm all for that, but I understand that the goal is to bake an automated `pep8` check into the build cycle. If we're OK with excluding this file from that check, then we can absolutely leave this file alone.

But that raises another question: Will we accept any contributions at all directly to cloudpickle, or is it basically frozen as is?