Fix mock import statements

As we dropped Python 2 support we don't have to use the third party
mock library with Python 2 compatibility and can move to Python 3
standard library. The change updates the import statements to reflect
that.

Change-Id: I39d9410ae43a2d9e6f6d6eaf74d06a82f2c33524

lower-constraints.txt

@@ -30,7 +30,6 @@ keystoneauth1==3.4.0
 linecache2==1.0.0
 MarkupSafe==1.0
 mccabe==0.2.1
-mock==2.0.0
 monotonic==0.6
 mox3==0.20.0
 msgpack-python==0.4.0

monascaclient/tests/test_client.py

@@ -14,9 +14,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from unittest import mock
 import warnings
 
-import mock
 from oslotest import base
 
 from monascaclient import client

monascaclient/tests/test_shell.py

@@ -14,7 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import mock
+from unittest import mock
 
 from oslotest import base
 

monascaclient/tests/v2_0/shell/test_alarm_definitions.py

@@ -12,7 +12,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
+from unittest import mock
 
 from oslotest import base
 

monascaclient/tests/v2_0/shell/test_metrics.py

@@ -12,7 +12,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
+from unittest import mock
 
 from oslotest import base
 

monascaclient/tests/v2_0/shell/test_notification_types.py

@@ -12,7 +12,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
+from unittest import mock
 
 from oslotest import base
 

monascaclient/tests/v2_0/shell/test_notifications.py

@@ -12,7 +12,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
+from unittest import mock
 
 from oslotest import base