[SPARK-21551][PYTHON] Increase timeout for PythonRDD.serveIterator

## What changes were proposed in this pull request?

This modification increases the timeout for `serveIterator` (which is not dynamically configurable). This fixes timeout issues in pyspark when using `collect` and similar functions, in cases where Python may take more than a couple seconds to connect.

See https://issues.apache.org/jira/browse/SPARK-21551

## How was this patch tested?

Ran the tests.

cc rxin

Author: peay <peay@protonmail.com>

Closes #18752 from peay/spark-21551.