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: Ieb1746b1ad7571cb87f5b1e7a6876ce72e6b76c3
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

lower-constraints.txt

@@ -20,7 +20,6 @@ jsonschema==2.6.0
 keystoneauth1==3.4.0
 linecache2==1.0.0
 mccabe==0.2.1
-mock==2.0.0
 monotonic==0.6
 mox3==0.20.0
 netaddr==0.7.18

os_client_config/tests/test_cloud_config.py

@@ -11,10 +11,10 @@
 # under the License.
 
 import copy
+from unittest import mock
 
 from keystoneauth1 import exceptions as ksa_exceptions
 from keystoneauth1 import session as ksa_session
-import mock
 
 from openstack.config import cloud_region
 

test-requirements.txt

@@ -8,7 +8,6 @@ coverage!=4.4,>=4.0 # Apache-2.0
 extras>=1.0.0 # MIT
 fixtures>=3.0.0 # Apache-2.0/BSD
 jsonschema>=2.6.0 # MIT
-mock>=2.0.0 # BSD
 python-glanceclient>=2.8.0 # Apache-2.0
 python-subunit>=1.0.0 # Apache-2.0/BSD
 oslotest>=3.2.0 # Apache-2.0