[SPARK-29668][PYTHON] Add a deprecation warning for Python 3.4 and 3.5

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to show a warning for deprecated Python 3.4 and 3.5 in Pyspark.

Why are the changes needed?

It's officially deprecated.

Does this PR introduce any user-facing change?

Yes, it shows a warning message for Python 3.4 and 3.5:

...
Using Spark's default log4j profile: org/apache/spark/log4j-defaults.properties
Setting default log level to "WARN".
To adjust logging level use sc.setLogLevel(newLevel). For SparkR, use setLogLevel(newLevel).
/.../spark/python/pyspark/context.py:220: DeprecationWarning: Support for Python 2 and Python 3 prior to version 3.6 is deprecated as of Spark 3.0. See also the plan for dropping Python 2 support at https://spark.apache.org/news/plan-for-dropping-python-2-support.html.
  DeprecationWarning)
...

How was this patch tested?

Manually tested.

[HyukjinKwon]

cc @dongjoon-hyun

[SparkQA]

Test build #112984 has finished for PR 26335 at commit 9b9ae00.

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

[dongjoon-hyun]

+1, LGTM. Thank you, @HyukjinKwon .
Merged to master.