[SPARK-16266][SQL][STREAING] Moved DataStreamReader/Writer from pyspark.sql to pyspark.sql.streaming

## What changes were proposed in this pull request?

- Moved DataStreamReader/Writer from pyspark.sql to pyspark.sql.streaming to make them consistent with scala packaging
- Exposed the necessary classes in sql.streaming package so that they appear in the docs
- Added pyspark.sql.streaming module to the docs

## How was this patch tested?
- updated unit tests.
- generated docs for testing visibility of pyspark.sql.streaming classes.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #13955 from tdas/SPARK-16266.

1 files changed, 2 insertions, 1 deletions

diff --git a/python/pyspark/sql/session.py b/python/pyspark/sql/session.py
index b4152a34ad..55f86a16f5 100644
--- a/python/pyspark/sql/session.py
+++ b/python/pyspark/sql/session.py
@@ -31,7 +31,8 @@ from pyspark.rdd import RDD, ignore_unicode_prefix
 from pyspark.sql.catalog import Catalog
 from pyspark.sql.conf import RuntimeConfig
 from pyspark.sql.dataframe import DataFrame
-from pyspark.sql.readwriter import DataFrameReader, DataStreamReader
+from pyspark.sql.readwriter import DataFrameReader
+from pyspark.sql.streaming import DataStreamReader
 from pyspark.sql.types import Row, DataType, StringType, StructType, _verify_type, \
     _infer_schema, _has_nulltype, _merge_type, _create_converter, _parse_datatype_string
 from pyspark.sql.utils import install_exception_handler