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: Iccf78a04a66dcef383d9e38ac3990f3c838bdf84

ironic_inspector/test/unit/test_plugins_base_physnet.py

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

ironic_inspector/test/unit/test_plugins_physnet_cidr_map.py

@@ -11,7 +11,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import mock
+from unittest import mock
+
 from openstack import exceptions
 from oslo_config import cfg
 

lower-constraints.txt

@@ -49,7 +49,6 @@ kombu==4.6.8
 linecache2==1.0.0
 Mako==1.1.2
 MarkupSafe==1.1.1
-mock==3.0.0
 monotonic==1.5
 mox3==1.0.0
 msgpack==1.0.0