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.

Change-Id: Iaf13c7dc3cf908d3755116ac861f732c220e324d

lower-constraints.txt

@@ -40,7 +40,6 @@ 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

magnumclient/tests/osc/unit/v1/test_cluster_templates.py

@@ -14,8 +14,8 @@
 #
 
 import copy
-from mock import call
 from unittest import mock
+from unittest.mock import call
 
 from magnumclient.exceptions import InvalidAttribute
 from magnumclient.osc.v1 import cluster_templates as osc_ct

magnumclient/tests/osc/unit/v1/test_clusters.py

@@ -24,7 +24,7 @@ try:
     from StringIO import StringIO
 except ImportError:
     from io import StringIO
-from mock import call
+from unittest.mock import call
 
 from magnumclient import exceptions
 from magnumclient.osc.v1 import clusters as osc_clusters

magnumclient/tests/osc/unit/v1/test_nodegroups.py

@@ -14,8 +14,8 @@
 #    under the License.
 
 import copy
-from mock import call
 from unittest import mock
+from unittest.mock import call
 
 from magnumclient.osc.v1 import nodegroups as osc_nodegroups
 from magnumclient.tests.osc.unit.v1 import fakes as magnum_fakes