Added the mock test dependency

It's a shame that Tox is unable (or unwilling) to get test
dependencies from package meta data. :(

tox.ini

@@ -20,6 +20,7 @@ deps =
     zope.interface
     zope.testing
     zope.testrunner
+    mock
 
 [testenv:simple]
 # Test that 'setup.py test' works