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: I8c269117dba546ee2c75685dbd29756407488aaf

os_testr/tests/test_ostestr.py

@@ -19,7 +19,7 @@ test_os_testr
 Tests for `os_testr` module.
 """
 import io
-import mock
+from unittest import mock
 
 from os_testr import ostestr as os_testr
 from os_testr.tests import base

os_testr/tests/test_regex_builder.py

@@ -13,7 +13,7 @@
 # under the License.
 
 import io
-import mock
+from unittest import mock
 
 from os_testr import regex_builder as os_testr
 from os_testr.tests import base

os_testr/tests/test_subunit_trace.py

@@ -18,11 +18,11 @@ import io
 import os
 import subprocess
 import sys
+from unittest.mock import patch
 
 from ddt import data
 from ddt import ddt
 from ddt import unpack
-from mock import patch
 import six
 
 from os_testr import subunit_trace

os_testr/tests/testlist_builder.py

@@ -12,8 +12,8 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
 import re
+from unittest import mock
 
 import six