## What changes were proposed in this pull request?
This PR explicitly imports the missing `warnings` in `flume.py`.
## How was this patch tested?
Manually tested.
```python
>>> import warnings
>>> warnings.simplefilter('always', DeprecationWarning)
>>> from pyspark.streaming import flume
>>> flume.FlumeUtils.createStream(None, None, None)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/.../spark/python/pyspark/streaming/flume.py", line 60, in createStream
warnings.warn(
NameError: global name 'warnings' is not defined
```
```python
>>> import warnings
>>> warnings.simplefilter('always', DeprecationWarning)
>>> from pyspark.streaming import flume
>>> flume.FlumeUtils.createStream(None, None, None)
/.../spark/python/pyspark/streaming/flume.py:65: DeprecationWarning: Deprecated in 2.3.0. Flume support is deprecated as of Spark 2.3.0. See SPARK-22142.
DeprecationWarning)
...
```
Author: hyukjinkwon <gurwls223@gmail.com>
Closes#20110 from HyukjinKwon/SPARK-22313-followup.
796e48c60e