Drop use of unittest2

unittest2 was needed for Python version <= 2.6, so it has not been
needed for quite some time now. See unittest2 note on:

https://docs.python.org/2.7/library/unittest.html

This replaces unittest2 in favor of the standard unittest package.

Change-Id: Idd5df4c8068810bbbd2bd5058ca38df9f5416a62
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

lower-constraints.txt

@@ -65,6 +65,5 @@ systemd-python==234
 testrepository==0.0.20
 testtools==2.3.0
 traceback2==1.4.0
-unittest2==1.1.0
 urllib3==1.22
 wrapt==1.10.11

oslo_log/tests/unit/test_versionutils.py

@@ -13,11 +13,12 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import unittest
+
 import mock
 from oslotest import base as test_base
 import six
 from testtools import matchers
-import unittest2
 
 from oslo_log import versionutils
 
@@ -247,7 +248,7 @@ class DeprecatedTestCase(test_base.BaseTestCase):
                                as_of='Juno',
                                remove_in='Kilo')
 
-    @unittest2.skipIf(
+    @unittest.skipIf(
         six.PY3,
         'Deprecated exception detection does not work for Python 3')
     @mock.patch('oslo_log.versionutils.report_deprecated_feature')