Add version annotation for PySequence_ITEM().

Doc/api/abstract.tex

@@ -770,6 +770,7 @@ determination.
   Macro form of \cfunction{PySequence_GetItem()} but without checking
   that \cfunction{PySequence_Check(\var{o})} is true and without
   adjustment for negative indices.
+  \versionadded{2.3}
 \end{cfuncdesc}
 
 \begin{cfuncdesc}{int}{PySequence_Fast_GET_SIZE}{PyObject *o}