[SPARK-26105][PYTHON] Clean unittest2 imports up that were added for Python 2.6 before

[HyukjinKwon]

What changes were proposed in this pull request?

Currently, some of PySpark tests sill assume the tests could be ran in Python 2.6 by importing unittest2. For instance:

if sys.version_info[:2] <= (2, 6):
    try:
        import unittest2 as unittest
    except ImportError:
        sys.stderr.write('Please install unittest2 to test with Python 2.6 or earlier')
        sys.exit(1)
else:
    import unittest

While I am here, I removed some of unused imports and reordered imports per PEP 8.

We officially dropped Python 2.6 support a while ago and started to discuss about Python 2 drop. It's better to remove them out.

How was this patch tested?

Manually tests, and existing tests via Jenkins.

[HyukjinKwon]

cc @BryanCutler.

BTW, Bryan, do you have some time to work on the has_numpy stuff that we talked about before? I was thinking we should specify the package version and produce similar skip messages like we do in Padnas and Arrow:

spark/python/pyspark/testing/sqlutils.py

Lines 30 to 55 in a7a331d

	pandas_requirement_message = None
	try:
	from pyspark.sql.utils import require_minimum_pandas_version
	require_minimum_pandas_version()
	except ImportError as e:
	# If Pandas version requirement is not satisfied, skip related tests.
	pandas_requirement_message = _exception_message(e)
	pyarrow_requirement_message = None
	try:
	from pyspark.sql.utils import require_minimum_pyarrow_version
	require_minimum_pyarrow_version()
	except ImportError as e:
	# If Arrow version requirement is not satisfied, skip related tests.
	pyarrow_requirement_message = _exception_message(e)
	test_not_compiled_message = None
	try:
	from pyspark.sql.utils import require_test_compiled
	require_test_compiled()
	except Exception as e:
	test_not_compiled_message = _exception_message(e)
	have_pandas = pandas_requirement_message is None
	have_pyarrow = pyarrow_requirement_message is None
	test_compiled = test_not_compiled_message is None

I'll try to work on this towards the end of the next week if you're busy.

[SparkQA]

Test build #98973 has finished for PR 23077 at commit ed834f5.

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

[BryanCutler]

LGTM, thanks @HyukjinKwon !

[BryanCutler]

Oops, actually I think there is one more here https://github.com/apache/spark/blob/master/python/pyspark/testing/mllibutils.py#L20

Other than that, looks good

[BryanCutler]

BTW, Bryan, do you have some time to work on the has_numpy stuff

Yup, I can do that

[BryanCutler]

Oh, I think the PR title should be SPARK-26105 too

[SparkQA]

Test build #98985 has finished for PR 23077 at commit a188076.

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

[HyukjinKwon]

Merged to master.

Thanks for reviewing this, @BryanCutler and @srowen.