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: Icac647c8e89cd94cae69e357b009d676b1c744f7
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==0.6.0
 netaddr==0.7.18

oslo_privsep/tests/test_capabilities.py

@@ -12,7 +12,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import mock
+from unittest import mock
 
 from oslotest import base
 

oslo_privsep/tests/test_daemon.py

@@ -16,10 +16,10 @@ import copy
 import fixtures
 import functools
 import logging as pylogging
-import mock
 import platform
 import sys
 import time
+from unittest import mock
 
 from oslo_log import formatters
 from oslo_log import log as logging

oslo_privsep/tests/test_priv_context.py

@@ -19,8 +19,8 @@ import pipes
 import platform
 import sys
 import tempfile
+from unittest import mock
 
-import mock
 import testtools
 
 from oslo_privsep import daemon

test-requirements.txt

@@ -4,7 +4,6 @@
 
 hacking>=1.1.0,<1.2.0 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
-mock>=2.0.0 # BSD
 fixtures>=3.0.0 # Apache-2.0/BSD
 stestr>=2.0.0 # Apache-2.0