[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/dataframe.py b/python/pyspark/sql/dataframe.py
index 4f13307820..e44b01bba9 100644
--- a/python/pyspark/sql/dataframe.py
+++ b/python/pyspark/sql/dataframe.py
@@ -33,7 +33,8 @@ from pyspark.storagelevel import StorageLevel
 from pyspark.traceback_utils import SCCallSiteSync
 from pyspark.sql.types import _parse_datatype_json_string
 from pyspark.sql.column import Column, _to_seq, _to_list, _to_java_column
-from pyspark.sql.readwriter import DataFrameWriter, DataStreamWriter
+from pyspark.sql.readwriter import DataFrameWriter
+from pyspark.sql.streaming import DataStreamWriter
 from pyspark.sql.types import *
 
 __all__ = ["DataFrame", "DataFrameNaFunctions", "DataFrameStatFunctions"]