Fix sphinx8 script used by pep8

We wrap doc8 to register the directives we use in our documentation.
Previously the 'app' argument was passed as None, however sphinx has
started to use the argument.

This change uses a mock object since we don't need to use the
application object.

Change-Id: Id9e8d5f6d09f14d294cd493538780456f98c7dbe