change Py3k backquote warning to a SyntaxWarning and add a test
diff --git a/Python/ast.c b/Python/ast.c
index 0bc7595..863906f 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -1364,7 +1364,7 @@
     case BACKQUOTE: { /* repr */
         expr_ty expression;
         if (Py_Py3kWarningFlag) {
-            if (PyErr_WarnExplicit(PyExc_DeprecationWarning,
+            if (PyErr_WarnExplicit(PyExc_SyntaxWarning,
                                    "backquote not supported in 3.x; use repr()",
                                    c->c_filename, LINENO(n),
                                    NULL, NULL)) {