[SPARK-14555] First cut of Python API for Structured Streaming

## What changes were proposed in this pull request?

This patch provides a first cut of python APIs for structured streaming. This PR provides the new classes:
 - ContinuousQuery
 - Trigger
 - ProcessingTime
in pyspark under `pyspark.sql.streaming`.

In addition, it contains the new methods added under:
 -  `DataFrameWriter`
     a) `startStream`
     b) `trigger`
     c) `queryName`

 -  `DataFrameReader`
     a) `stream`

 - `DataFrame`
    a) `isStreaming`

This PR doesn't contain all methods exposed for `ContinuousQuery`, for example:
 - `exception`
 - `sourceStatuses`
 - `sinkStatus`

They may be added in a follow up.

This PR also contains some very minor doc fixes in the Scala side.

## How was this patch tested?

Python doc tests

TODO:
 - [ ] verify Python docs look good

Author: Burak Yavuz <brkyvz@gmail.com>
Author: Burak Yavuz <burak@databricks.com>

Closes #12320 from brkyvz/stream-python.

1 files changed, 2 insertions, 2 deletions

diff --git a/python/pyspark/ml/tuning.py b/python/pyspark/ml/tuning.py
index 5ac539edde..ef14da488e 100644
--- a/python/pyspark/ml/tuning.py
+++ b/python/pyspark/ml/tuning.py
@@ -19,11 +19,11 @@ import itertools
 import numpy as np
 
 from pyspark import SparkContext
-from pyspark import since
+from pyspark import since, keyword_only
 from pyspark.ml import Estimator, Model
 from pyspark.ml.param import Params, Param, TypeConverters
 from pyspark.ml.param.shared import HasSeed
-from pyspark.ml.util import keyword_only, JavaMLWriter, JavaMLReader, MLReadable, MLWritable
+from pyspark.ml.util import JavaMLWriter, JavaMLReader, MLReadable, MLWritable
 from pyspark.ml.wrapper import JavaParams
 from pyspark.sql.functions import rand
 from pyspark.mllib.common import inherit_doc, _py2java