Remove unittest2 imports (#9171)

They were only needed for python versions <= 2.6

Related: #6658

python/google/protobuf/internal/_parameterized.py

@@ -148,10 +148,7 @@ __author__ = 'tmarek@google.com (Torsten Marek)'
 import functools
 import re
 import types
-try:
-  import unittest2 as unittest
-except ImportError:
-  import unittest
+import unittest
 import uuid
 
 try:

python/google/protobuf/internal/generator_test.py

@@ -39,10 +39,7 @@ further ensures that we can use Python protocol message objects as we expect.
 
 __author__ = 'robinson@google.com (Will Robinson)'
 
-try:
-  import unittest2 as unittest  #PY26
-except ImportError:
-  import unittest
+import unittest
 
 from google.protobuf.internal import test_bad_identifiers_pb2
 from google.protobuf import unittest_custom_options_pb2

python/google/protobuf/internal/proto_builder_test.py

@@ -31,10 +31,7 @@
 """Tests for google.protobuf.proto_builder."""
 
 import collections
-try:
-  import unittest2 as unittest
-except ImportError:
-  import unittest
+import unittest
 
 from google.protobuf import descriptor_pb2  # pylint: disable=g-import-not-at-top
 from google.protobuf import descriptor