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

## 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:

```python
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.

Closes apache#23077 from HyukjinKwon/SPARK-26105.

Lead-authored-by: hyukjinkwon <gurwls223@apache.org>
Co-authored-by: Bryan Cutler <cutlerb@gmail.com>
Signed-off-by: hyukjinkwon <gurwls223@apache.org>

Author: 2 people

python/pyspark/ml/tests/test_algorithms.py

@@ -16,17 +16,8 @@
 #
 
 from shutil import rmtree
-import sys
 import tempfile
-
-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
+import unittest
 
 import numpy as np
 

python/pyspark/ml/tests/test_base.py

@@ -15,15 +15,7 @@
 # limitations under the License.
 #
 
-import sys
-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
+import unittest
 
 from pyspark.sql.types import DoubleType, IntegerType
 from pyspark.testing.mlutils import MockDataset, MockEstimator, MockUnaryTransformer, \

python/pyspark/ml/tests/test_evaluation.py

@@ -15,15 +15,7 @@
 # limitations under the License.
 #
 
-import sys
-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
+import unittest
 
 import numpy as np
 

python/pyspark/ml/tests/test_feature.py

@@ -17,14 +17,7 @@
 #
 
 import sys
-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
+import unittest
 
 if sys.version > '3':
     basestring = str

python/pyspark/ml/tests/test_image.py

@@ -14,15 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
-import sys
-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
+import unittest
 
 import py4j
 

python/pyspark/ml/tests/test_linalg.py

@@ -15,17 +15,9 @@
 # limitations under the License.
 #
 
-import sys
-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
-
+import unittest
 import array as pyarray
+
 from numpy import arange, array, array_equal, inf, ones, tile, zeros
 
 from pyspark.ml.linalg import DenseMatrix, DenseVector, MatrixUDT, SparseMatrix, SparseVector, \

python/pyspark/ml/tests/test_param.py

@@ -19,17 +19,7 @@
 import inspect
 import sys
 import array as pyarray
-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
-
-if sys.version > '3':
-    xrange = range
+import unittest
 
 import numpy as np
 
@@ -45,6 +35,10 @@
 from pyspark.testing.mlutils import check_params, PySparkTestCase, SparkSessionTestCase
 
 
+if sys.version > '3':
+    xrange = range
+
+
 class ParamTypeConversionTests(PySparkTestCase):
     """
     Test that param type conversion happens.

python/pyspark/ml/tests/test_persistence.py

@@ -17,16 +17,8 @@
 
 import json
 from shutil import rmtree
-import sys
 import tempfile
-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
+import unittest
 
 from pyspark.ml import Transformer
 from pyspark.ml.classification import DecisionTreeClassifier, LogisticRegression, OneVsRest, \

python/pyspark/ml/tests/test_pipeline.py

@@ -14,15 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
-import sys
-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
+import unittest
 
 from pyspark.ml.pipeline import Pipeline
 from pyspark.testing.mlutils import MockDataset, MockEstimator, MockTransformer, PySparkTestCase

python/pyspark/ml/tests/test_stat.py

@@ -15,15 +15,7 @@
 # limitations under the License.
 #
 
-import sys
-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
+import unittest
 
 from pyspark.ml.linalg import Vectors
 from pyspark.ml.stat import ChiSquareTest