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: I2f8067eeb2aec62f098bfbb3770a67f1876ce7a2
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

lower-constraints.txt

@@ -19,7 +19,6 @@ keystoneauth1==3.4.0
 linecache2==1.0.0
 MarkupSafe==1.0
 mccabe==0.2.1
-mock==3.0.0
 mox3==0.20.0
 mypy==0.720
 netaddr==0.7.18

oslo_config/tests/test_cfg.py

@@ -21,9 +21,9 @@ import os
 import shutil
 import sys
 import tempfile
+from unittest import mock
 
 import fixtures
-import mock
 from oslotest import base
 import testscenarios
 

oslo_config/tests/test_generator.py

@@ -15,9 +15,9 @@
 import io
 import sys
 import textwrap
+from unittest import mock
 
 import fixtures
-import mock
 from oslotest import base
 import tempfile
 import testscenarios

oslo_config/tests/test_sphinxconfiggen.py

@@ -10,7 +10,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import mock
+from unittest import mock
+
 from oslotest import base
 
 from oslo_config import sphinxconfiggen

oslo_config/tests/test_sphinxext.py

@@ -11,8 +11,8 @@
 #    under the License.
 
 import textwrap
+from unittest import mock
 
-import mock
 from oslotest import base
 
 from oslo_config import cfg

oslo_config/tests/test_validator.py

@@ -12,7 +12,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import mock
+from unittest import mock
+
 from oslotest import base
 
 from oslo_config import cfg

test-requirements.txt

@@ -21,9 +21,6 @@ oslo.log>=3.36.0  # Apache-2.0
 #  deps = {[testenv]deps} coverage
 coverage!=4.4,>=4.0 # Apache-2.0
 
-# we can switch to unittest.mock once we drop support for Python 3.6 as that
-# includes https://bugs.python.org/issue32933
-mock>=3.0.0 # BSD
 requests_mock>=1.5.0 # Apache-2.0
 
 # Bandit security code scanner