tests: use unittest.mock when available

Python3 includes unittest.mock so the third-party mock is not strictly
required when using python3.

Closes git-cola#569
Suggested-by: Ilya Tumaykin <itumaykin@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>

share/doc/git-cola/relnotes.rst

@@ -6,6 +6,7 @@ Releases
     :titlesonly:
 
     relnotes/unreleased
+    relnotes/v2.7
     relnotes/v2.6
     relnotes/v2.5
     relnotes/v2.4

share/doc/git-cola/relnotes/v2.7.rst

@@ -0,0 +1,12 @@
+.. _v2.7:
+
+git-cola v2.7 (pre-release)
+===========================
+
+Development
+-----------
+
+* The `unittest.mock` module is now used instead of the original `mock` module
+  when running the `git cola` test suite using Python3.
+
+  https://github.com/git-cola/git-cola/issues/569

test/git_test.py

@@ -8,7 +8,10 @@
 import time
 import unittest
 
-from mock import patch
+try:
+    from unittest.mock import patch
+except ImportError:
+    from mock import patch
 
 from cola import git
 from cola.compat import WIN32