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: I792a17f4b2667713503e6bc10cfabed9905d8535
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

lower-constraints.txt

@@ -24,7 +24,6 @@ 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
 netaddr==0.7.18

oslo_concurrency/tests/unit/test_lockutils.py

@@ -21,8 +21,8 @@ import sys
 import tempfile
 import threading
 import time
+from unittest import mock
 
-import mock
 from oslo_config import cfg
 from oslotest import base as test_base
 import six

oslo_concurrency/tests/unit/test_processutils.py

@@ -26,9 +26,9 @@ import stat
 import subprocess
 import sys
 import tempfile
+from unittest import mock
 
 import fixtures
-import mock
 from oslotest import base as test_base
 import six