Use unittest.mock wherever it's available

Since python 3.3 `mock` is part of standard library. Let's try to use
that before looking for third-party installation of it.

test/EditCommandTest.py

@@ -15,7 +15,12 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import unittest
-import mock
+
+try:
+    from unittest import mock
+except ImportError:
+    import mock
+
 from six import u
 import os