Fixed #13110 -- Added support for multiple enclosures in Atom feeds.

The ``item_enclosures`` hook returns a list of ``Enclosure`` objects which is
then used by the feed builder. If the feed is a RSS feed, an exception is
raised as RSS feeds don't allow multiple enclosures per feed item.

The ``item_enclosures`` hook defaults to an empty list or, if the
``item_enclosure_url`` hook is defined, to a list with a single ``Enclosure``
built from the ``item_enclosure_url``, ``item_enclosure_length``, and
``item_enclosure_mime_type`` hooks.

docs/internals/deprecation.txt

@@ -94,6 +94,9 @@ details on these changes.
 * The ``callable_obj`` keyword argument to
   ``SimpleTestCase.assertRaisesMessage()`` will be removed.
 
+* The ``enclosure`` keyword argument to ``SyndicationFeed.add_item()`` will be
+  removed.
+
 .. _deprecation-removed-in-1.10:
 
 1.10