[Docs] Fix mocking external modules

With move to GTK3 needed to update the mocking of external modules.

There is a new autodoc option `autodoc_mock_imports` so use this instead
of the custom mock class.

There are some build warnings output using autodoc mock:

    TypeError: unsupported operand type(s) for |: '_MockObject' and '_MockObject'

Will resolve these later as the build passes.

docs/source/conf.py

@@ -95,12 +95,6 @@ autodoc_mock_imports = [
     'PIL',
     'libtorrent',
     'psyco',
-    'pygtk',
-    'gtk',
-    'gobject',
-    'pango',
-    'cairo',
-    'pangocairo',
     'gi',
     'curses',
     'win32api',