"Fix" an exception formatting issue on Py2 (#5018)

Cython/Utility/CythonFunction.c

@@ -660,9 +660,17 @@ static PyObject *__Pyx_CyFunction_CallAsMethod(PyObject *func, PyObject *args, P
         self = PyTuple_GetItem(args, 0);
         if (unlikely(!self)) {
             Py_DECREF(new_args);
+#if PY_MAJOR_VERSION > 2
             PyErr_Format(PyExc_TypeError,
                          "unbound method %.200S() needs an argument",
                          cyfunc->func_qualname);
+#else
+            // %S doesn't work in PyErr_Format on Py2 and replicating
+            // the formatting seems more trouble than it's worth
+            // (so produce a less useful error message).
+            PyErr_SetString(PyExc_TypeError,
+                            "unbound method needs an argument");
+#endif
             return NULL;
         }