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: I8ca155bda0939524a4b5d940ac719655fde9fc92
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

reno/tests/test_cache.py

@@ -13,10 +13,11 @@
 # under the License.
 
 import collections
+from unittest import mock
+
 import fixtures
 import textwrap
 
-import mock
 
 from reno import cache
 from reno import config

reno/tests/test_config.py

@@ -13,6 +13,7 @@
 # under the License.
 import argparse
 import os
+from unittest import mock
 
 import fixtures
 
@@ -21,8 +22,6 @@ from reno import defaults
 from reno import main
 from reno.tests import base
 
-import mock
-
 
 class TestConfig(base.TestCase):
     EXAMPLE_CONFIG = """

reno/tests/test_create.py

@@ -12,9 +12,10 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+from unittest import mock
+
 import fixtures
 import io
-import mock
 
 from reno import create
 from reno.tests import base

reno/tests/test_formatter.py

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

reno/tests/test_loader.py

@@ -14,9 +14,9 @@
 
 import logging
 import textwrap
+from unittest import mock
 
 import fixtures
-import mock
 import yaml
 
 from reno import config

reno/tests/test_scanner.py

@@ -13,6 +13,7 @@
 # under the License.
 
 from __future__ import unicode_literals
+from unittest import mock
 
 import itertools
 import logging
@@ -25,7 +26,6 @@ import unittest
 from dulwich import diff_tree
 from dulwich import objects
 import fixtures
-import mock
 from testtools.content import text_content
 
 from reno import config

reno/tests/test_utils.py

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

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.
 
-mock>=1.2
 
 coverage!=4.4,>=4.0 # Apache-2.0
 python-subunit>=0.0.18