Use unittest.mock instead of third party mock

Now that we no longer support py27, we can use the standard library
unittest.mock module instead of the third party mock lib.

Change-Id: I0ec3d1c0f53bf859ea1ec115ebb47f1d5f3b217c
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

lower-constraints.txt

@@ -21,7 +21,6 @@ keystoneauth1==3.4.0
 linecache2==1.0.0
 MarkupSafe==1.0
 mccabe==0.2.1
-mock==2.0.0
 mox3==0.20.0
 msgpack-python==0.4.0
 netaddr==0.7.18

oslo_reports/tests/test_guru_meditation_report.py

@@ -20,11 +20,11 @@ import re
 import signal
 import sys
 import threading
+from unittest import mock
 
 # needed to get greenthreads
 import fixtures
 import greenlet
-import mock
 from oslotest import base
 import six
 

oslo_reports/tests/test_openstack_generators.py

@@ -14,9 +14,9 @@
 #    under the License.
 
 import threading
+from unittest import mock
 
 import greenlet
-import mock
 from oslo_config import cfg
 from oslotest import base
 import six

oslo_reports/tests/test_views.py

@@ -13,8 +13,8 @@
 #    under the License.
 
 import copy
+from unittest import mock
 
-import mock
 from oslotest import base
 import six