SF patch #581396, Canvas "select_item" always returns None

Return the selected item, if there is any.
diff --git a/Lib/lib-tk/Tkinter.py b/Lib/lib-tk/Tkinter.py
index c5bf136..573d71d 100644
--- a/Lib/lib-tk/Tkinter.py
+++ b/Lib/lib-tk/Tkinter.py
@@ -2103,7 +2103,7 @@
         self.tk.call(self._w, 'select', 'from', tagOrId, index)
     def select_item(self):
         """Return the item which has the selection."""
-        self.tk.call(self._w, 'select', 'item')
+        return self.tk.call(self._w, 'select', 'item') or None
     def select_to(self, tagOrId, index):
         """Set the variable end of a selection in item TAGORID to INDEX."""
         self.tk.call(self._w, 'select', 'to', tagOrId, index)