Extension.setup() should raise NotImplementedError

[adamcik]

Having just wasted at least an hour hunting down why spotify wasn't working because of this I'm strongly for this breaking loudly when you have an old pre setup extension for some reason. Yes this shouldn't happen often, if at all, but this was a pain in the ass to debug (mopidy deps "lies" in this case btw).

Problem turned out to be that the 1.2.0 wheel was actually a 1.0.3 codebase, so it installs just fine. But since it has get_backends instead of setup it never gets used...