docs: Require Pykka for docs building (fixes #1500)

Pykka is a pure Python dependency, so we might as well depend on it for
docs building so that :autoclass: works on Pykka subclasses in the docs.

docs/changelog.rst

@@ -49,6 +49,11 @@ Bug fix release.
 - Stream: Fix milliseconds vs seconds mistake in timeout handling.
   (Fixes: :issue:`1521`, PR: :issue:`1522`)
 
+- Docs: Fix the rendering of :class:`mopidy.core.Core` and
+  :class:`mopidy.audio.Audio` docs. This should also contribute towards making
+  the Mopidy Debian package build bit-by-bit reproducible. (Fixes:
+  :issue:`1500`)
+
 
 v2.0.0 (2016-02-15)
 ===================

docs/conf.py

@@ -40,7 +40,6 @@ MOCK_MODULES = [
     'dbus.mainloop.glib',
     'dbus.service',
     'mopidy.internal.gi',
-    'pykka',
 ]
 for mod_name in MOCK_MODULES:
     sys.modules[mod_name] = Mock()

docs/requirements.txt

@@ -1,3 +1,4 @@
 Sphinx >= 1.0
-sphinx_rtd_theme
 pygraphviz
+Pykka >= 1.1
+sphinx_rtd_theme