Use unittest.mock instead of mock

The mock third party library was needed for mock support in py2
runtimes. Since we now only support py36 and later, we can use the
standard lib unittest.mock module instead.

Bump horizon to 18.3.0 in lower-constraints.txt since it is the
first version with "Use unittest.mock instead of third party mock"
change [1]

[1] 7eeb9e14f9

Change-Id: Ic64fffaeded18ac11ce438c77526e60a37aeaa1d

lower-constraints.txt

@@ -28,7 +28,7 @@ fixtures==3.0.0
 funcsigs==1.0.0
 futurist==1.2.0
 greenlet==0.4.10
-horizon==17.1.0
+horizon==18.3.0
 idna==2.6
 imagesize==0.7.1
 iso8601==0.1.11
@@ -42,7 +42,6 @@ kombu==4.0.0
 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

monitoring/alarmdefs/tests.py

@@ -14,7 +14,7 @@
 # under the License.
 
 from django.urls import reverse
-from mock import patch
+from unittest.mock import patch
 
 from monitoring.alarmdefs import constants
 from monitoring.alarmdefs import views

monitoring/alarms/tests.py

@@ -11,7 +11,7 @@
 #  under the License.
 
 from django.urls import reverse
-from mock import patch
+from unittest.mock import patch
 
 from monitoring.alarms import constants
 from monitoring.alarms import tables

monitoring/notifications/tests.py

@@ -11,7 +11,7 @@
 #  under the License.
 
 from django.urls import reverse
-from mock import patch
+from unittest.mock import patch
 
 from monitoring.notifications import constants
 from monitoring.test import helpers

monitoring/test/helpers.py

@@ -16,8 +16,8 @@ import unittest
 import warnings
 
 from django.core.handlers import wsgi
-import mock
 from openstack_dashboard.test import helpers
+from unittest import mock
 
 from monitoring.test.test_data import utils as test_data_utils
 

monitoring/test/test_alarm_tables.py

@@ -14,7 +14,7 @@
 
 from django.test import TestCase
 from django.test.utils import override_settings
-from mock import Mock
+from unittest.mock import Mock
 
 import monitoring.alarms.tables
 

monitoring/test/test_client.py

@@ -16,8 +16,8 @@
 
 from django.conf import settings as d_settings
 from django.test.utils import override_settings
-from mock import Mock
-from mock import patch
+from unittest.mock import Mock
+from unittest.mock import patch
 
 from monascaclient import client as mon_client
 from monascaclient import v2_0

test-requirements.txt

@@ -5,7 +5,6 @@
 hacking>=3.0.1,<3.1.0 # Apache-2.0
 
 coverage!=4.4,>=4.0 # Apache-2.0
-mock>=2.0.0 # BSD
 nodeenv>=0.9.4 # BSD
 selenium>=2.50.1 # Apache-2.0