du10/du11 need mock dependency

tox.ini

@@ -35,10 +35,12 @@ deps=
 
 [testenv:du10]
 deps=
+    mock
     docutils==0.10
     {[testenv]deps}
 
 [testenv:du11]
 deps=
+    mock
     docutils==0.11
     {[testenv]deps}