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: I5b6f25b1154b9d655843615ca8402fe4f73e923f
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

lower-constraints.txt

@@ -14,7 +14,6 @@ iso8601==0.1.11
 keystoneauth1==3.4.0
 linecache2==1.0.0
 mccabe==0.2.1
-mock==2.0.0
 mox3==0.20.0
 msgpack==0.5.2
 netaddr==0.7.18

oslo_serialization/tests/test_jsonutils.py

@@ -20,9 +20,9 @@ import io
 import ipaddress
 import itertools
 import json
+from unittest import mock
 from xmlrpc import client as xmlrpclib
 
-import mock
 import netaddr
 from oslo_i18n import fixture
 from oslotest import base as test_base

test-requirements.txt

@@ -2,7 +2,6 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 hacking>=1.1.0,<1.2.0 # Apache-2.0
-mock>=2.0.0 # BSD
 netaddr>=0.7.18 # BSD
 stestr>=2.0.0 # Apache-2.0