Use the standard library's copy of mock when it's available.

Change-Id: Id6a31157582efe47cc3a74e6658679c2bec14767

tests/test_command_helpers.py

@@ -13,8 +13,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+try:
+    from unittest import mock
+except ImportError:
+    import mock
+
 from six import StringIO
-import mock
 import testtools
 
 from swiftclient import command_helpers as h

tests/test_multithreading.py

@@ -15,7 +15,12 @@
 
 import sys
 import time
-import mock
+
+try:
+    from unittest import mock
+except ImportError:
+    import mock
+
 import testtools
 import threading
 import six

tests/test_swiftclient.py

@@ -14,8 +14,13 @@
 # limitations under the License.
 
 # TODO: More tests
-import mock
 import logging
+
+try:
+    from unittest import mock
+except ImportError:
+    import mock
+
 import six
 import socket
 import testtools