change Py3k backquote warning to a SyntaxWarning and add a test

Lib/test/test_py3kwarn.py

@@ -10,6 +10,12 @@ if not sys.py3kwarning:
 
 class TestPy3KWarnings(unittest.TestCase):
 
+    def test_backquote(self):
+        expected = 'backquote not supported in 3.x; use repr()'
+        with catch_warning() as w:
+            exec "`2`" in {}
+        self.assertWarning(None, w, expected)
+
     def test_type_inequality_comparisons(self):
         expected = 'type inequality comparisons not supported in 3.x'
         with catch_warning() as w:

Python/ast.c

@@ -1364,7 +1364,7 @@ ast_for_atom(struct compiling *c, const node *n)
     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)) {