[SPARK-3867][PySpark] ./python/run-tests failed when it run with Python 2.6 and unittest2 is not installed

./python/run-tests search a Python 2.6 executable on PATH and use it if available.
When using Python 2.6, it is going to import unittest2 module which is not a standard library in Python 2.6, so it fails with ImportError.

Author: cocoatomo <cocoatomo77@gmail.com>

Closes apache#2759 from cocoatomo/issues/3867-unittest2-import-error and squashes the following commits:

f068eb5 [cocoatomo] [SPARK-3867] ./python/run-tests failed when it run with Python 2.6 and unittest2 is not installed

Author: cocoatomo

python/pyspark/mllib/tests.py

@@ -25,7 +25,11 @@
 from numpy import array, array_equal
 
 if sys.version_info[:2] <= (2, 6):
-    import unittest2 as unittest
+    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
 

python/pyspark/tests.py

@@ -34,7 +34,11 @@
 from platform import python_implementation
 
 if sys.version_info[:2] <= (2, 6):
-    import unittest2 as unittest
+    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