Docs generation: mock the six module for autodoc

Currently, tox-edocs fails with the following error message:

Warning, treated as error:
autodoc: failed to import module 'shell' from module 'glanceclient'; the
following exception was raised:
No module named 'six'

We used to install six in the virtual environment through keystoneauth,
but keystoneauth no longer depends on six. Let's mock six completely in
the context of autodoc.

Change-Id: Ibcf0b9698227a2c22fe991b7516ba5dceabfc607

doc/source/conf.py

@@ -46,6 +46,9 @@ openstackdocs_bug_tag = ''
 # text edit cycles.
 # execute "export SPHINX_DEBUG=1" in your terminal to disable
 
+# TODO: remove this once no dependency uses six anymore
+autodoc_mock_imports = ['six']
+
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']