sphinx: Configure sys.path to allow autodoc to find Kazoo.

Previously, if Kazoo wasn't already in sys.path otherwise (e.g. if not
installed) sphinx autodoc wouldn't be able to find Kazoo for documenting
purposes because sphinx-build happens in a subdirectory adjacent to
Kazoo.

This adds the docs folder's parent to the path so that autodoc can find
the Kazoo module properly and do its stuff.