Comment 22 for bug 1210253

I can still see the contradictory code in horizon/__init__.py. Should that be fixed too?
I mean, if we decide that we do require Dashboard, Panel and PanelGroup, then maybe we should remove the try-except and warnings? If we decide that they are optional after all, then we should remove the asserts?

Also, why asserts, when using __all__ also makes flake8 shut up and actually is the idiomatic way of telling Python what those modules should me exposed.