[SPARK-22313][PYTHON][FOLLOWUP] Explicitly import warnings namespace in flume.py

[HyukjinKwon]

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.

>>> 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

>>> 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)
...

[HyukjinKwon]

cc @yhuai. Thank you for catching this.

[HyukjinKwon]

cc @ueshin too.

[yhuai]

Thank you! Let's also check the build result to make sure pyspark.streaming.tests.FlumePollingStreamTests is indeed triggered (I hit this issue while running this test).

[SparkQA]

Test build #85492 has finished for PR 20110 at commit 6b73dd8.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[ueshin]

LGTM for the change, but I'm not sure whether the test was indeed triggered or not.

[ueshin]

I confirmed the test came to pass after the patch in my local environment.

[ueshin]

Thanks! merging to master.

[HyukjinKwon]

Thank you @yhuai and @ueshin.