Switch from mock to unittest.mock

We were relying on oslotest using the mock library and we were not
declaring it in our test-requirements.txt file, but now they are using
unittest.mock instead [1], so our unit tests are now failing because the
mock library is missing.

Switch to unittest.mock that is present in Python 3 to resolve the
problem.

[1]: https://review.opendev.org/#/c/715704/

Change-Id: If79500534aed2ab992c423f106088905edcb9d7f

cinderlib/tests/unit/base.py

@@ -14,8 +14,8 @@
 #    under the License.
 
 
-import mock
 import unittest
+from unittest import mock
 
 from oslo_config import cfg
 import six

cinderlib/tests/unit/objects/test_connection.py

@@ -13,8 +13,9 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from unittest import mock
+
 import ddt
-import mock
 
 from cinderlib import exception
 from cinderlib import objects

cinderlib/tests/unit/objects/test_snapshot.py

@@ -13,7 +13,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import mock
+from unittest import mock
 
 from cinderlib import exception
 from cinderlib import objects

cinderlib/tests/unit/objects/test_volume.py

@@ -13,7 +13,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import mock
+from unittest import mock
 
 from cinder import objects as cinder_ovos
 

cinderlib/tests/unit/persistence/base.py

@@ -13,7 +13,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import mock
+from unittest import mock
+
 from oslo_config import cfg
 
 import cinderlib

cinderlib/tests/unit/persistence/test_memory.py

@@ -13,8 +13,9 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from unittest import mock
+
 from cinder import objects as ovos
-import mock
 
 import cinderlib
 from cinderlib import objects

cinderlib/tests/unit/test_cinderlib.py

@@ -15,9 +15,9 @@
 
 import collections
 import os
+from unittest import mock
 
 import ddt
-import mock
 from oslo_config import cfg
 
 import cinderlib

cinderlib/tests/unit/test_nos_brick.py

@@ -14,9 +14,9 @@
 #    under the License.
 
 import errno
+from unittest import mock
 
 import ddt
-import mock
 from os_brick import exception
 
 from cinderlib import nos_brick

cinderlib/tests/unit/utils.py

@@ -13,7 +13,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import mock
+from unittest import mock
 
 import cinderlib
 from cinderlib.persistence import base