[SPARK-16268][PYSPARK] SQLContext should import DataStreamReader

## What changes were proposed in this pull request?

Fixed the following error:
```
>>> sqlContext.readStream
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "...", line 442, in readStream
    return DataStreamReader(self._wrapped)
NameError: global name 'DataStreamReader' is not defined
```

## How was this patch tested?

The added test.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #13958 from zsxwing/fix-import.