Use unittest.mock instead of mock

The mock third party library was needed for mock support in py2
runtimes. Since we now only support py36 and later, we can use the
standard lib unittest.mock module instead.

Change-Id: I453515f347a59f4c6f60e0dcba2c327cf2a40426

lower-constraints.txt

@@ -20,7 +20,6 @@ linecache2==1.0.0
 lxml==3.4.1
 MarkupSafe==1.0
 mccabe==0.2.1
-mock==2.0.0
 openstackdocstheme==2.2.1
 pbr==2.0.0
 pep8==1.5.7

test-requirements.txt

@@ -13,6 +13,3 @@ pylint==1.7.1 # GPLv2
 reno>=3.1.0 # Apache-2.0
 openstackdocstheme>=2.2.1 # Apache-2.0
 stestr>=2.0.0 # Apache-2.0
-
-# mock object framework
-mock>=2.0.0 # BSD

test/test_pipelines.py

@@ -10,9 +10,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
 from sitemap.generator import pipelines
 import unittest
+from unittest import mock
 
 
 class TestSitemapItemExporter(unittest.TestCase):

test/test_sitemap_file.py

@@ -10,10 +10,10 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
 import scrapy
 from sitemap.generator.spiders import sitemap_file
 import unittest
+from unittest import mock
 
 
 class TestSitemapItem(unittest.TestCase):