Switch to unittest.mock

Since mock is a backport of the standard library, and we no longer
support Python 2, we can use the standard library, and drop one external
dependency.

Change-Id: I798c85f1581f4562908c10cd1b58134cdcb40281

git_review/tests/test_unit.py

@@ -21,9 +21,9 @@ import io
 import os
 import sys
 import textwrap
+from unittest import mock
 
 import fixtures
-import mock
 import testtools
 
 from git_review import cmd

test-requirements.txt

@@ -3,7 +3,6 @@
 # due to non SNI TLS connections being disabled.
 pbr ; python_version >= "3" and python_version <= "3.5"
 hacking>=2.0.0,<2.1.0
-mock
 fixtures>=0.3.14
 stestr>=2.2.0,<3.0.0
 testtools>=0.9.34