backend: Remove old backends API

As far as I know, Mopidy-VKontakte is the only backend extension that hasn't
been updated to use the new API yet. They have a PR ready for merging that does
the update for them.

docs/changelog.rst

@@ -10,6 +10,12 @@ v0.19.0 (UNRELEASED)
 
 Feature release.
 
+**Backend API**
+
+- Imports of the backend API from :mod:`mopidy.backends` no longer works. The
+  new API introuced in v0.18 is now required. Most extensions already use the
+  new API location.
+
 **MPD frontend**
 
 - Proper command tokenization for MPD requests. This replaces the old regex

mopidy/backends/__init__.py

@@ -1 +0,0 @@
-from __future__ import unicode_literals

mopidy/backends/base.py

@@ -1,17 +0,0 @@
-from __future__ import unicode_literals
-
-from mopidy.backend import (
-    Backend,
-    LibraryProvider as BaseLibraryProvider,
-    PlaybackProvider as BasePlaybackProvider,
-    PlaylistsProvider as BasePlaylistsProvider)
-
-
-# Make classes previously residing here available in the old location for
-# backwards compatibility with extensions targeting Mopidy < 0.18.
-__all__ = [
-    'Backend',
-    'BaseLibraryProvider',
-    'BasePlaybackProvider',
-    'BasePlaylistsProvider',
-]

mopidy/backends/dummy.py

@@ -1,5 +0,0 @@
-from __future__ import unicode_literals
-
-# Make classes previously residing here available in the old location for
-# backwards compatibility with extensions targeting Mopidy < 0.18.
-from mopidy.backend.dummy import *  # noqa

mopidy/backends/listener.py

@@ -1,8 +0,0 @@
-from __future__ import unicode_literals
-
-from mopidy.backend import BackendListener
-
-
-# Make classes previously residing here available in the old location for
-# backwards compatibility with extensions targeting Mopidy < 0.18.
-__all__ = ['BackendListener']