Merge pull request realpython#380 from sigmavirus24/add-mock-note

Add note about mock in Python 3.3

Author: sigmavirus24

docs/writing/tests.rst

@@ -251,7 +251,9 @@ the need to change any other code.
 mock
 ----
 
-mock is a library for testing in Python.
+mock is a library for testing in Python. Starting with Python 3.3, it is 
+available in the `standard library <http://docs.python.org/dev/library/unittest.mock`_.  For older versions of 
+python, simply:
 
 .. code-block:: console