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: I4df9a945e33c7b77c38d139195cab2fa0481209e
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

lower-constraints.txt

@@ -22,7 +22,6 @@ keystoneauth1==3.4.0
 linecache2==1.0.0
 MarkupSafe==1.0
 mccabe==0.2.1
-mock==2.0.0
 monotonic==1.4
 mox3==0.25.0
 msgpack==0.5.6

oslo_cache/tests/test_cache.py

@@ -15,9 +15,9 @@
 
 import copy
 import time
+from unittest import mock
 
 from dogpile.cache import proxy
-import mock
 from oslo_config import cfg
 from oslo_config import fixture as config_fixture
 from oslo_utils import uuidutils

oslo_cache/tests/test_connection_pool.py

@@ -12,8 +12,8 @@
 
 import threading
 import time
+from unittest import mock
 
-import mock
 import six
 from six.moves import queue
 import testtools

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>=3.0,<3.1.0 # Apache-2.0
-mock>=2.0.0 # BSD
 oslotest>=3.2.0 # Apache-2.0
 pifpaf>=0.10.0 # Apache-2.0
 # Bandit security code scanner