[SPARK-18888] partitionBy in DataStreamWriter in Python throws _to_seq not defined

## What changes were proposed in this pull request?

`_to_seq` wasn't imported.

## How was this patch tested?

Added partitionBy to existing write path unit test

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #16297 from brkyvz/SPARK-18888.