Added a warning when -3 is enabled and None is passed to filter as the first argument.

Python/bltinmodule.c

@@ -296,6 +296,13 @@ builtin_filter(PyObject *self, PyObject *args)
 		}
 
 		if (func == (PyObject *)&PyBool_Type || func == Py_None) {
+			if (Py_Py3kWarningFlag &&
+			    PyErr_Warn(PyExc_DeprecationWarning,
+				       "filter with None as a first argument "
+				       "is not supported in 3.x.  Use a list "
+				       "comprehension instead.") < 0)
+				return NULL;
+
 			ok = PyObject_IsTrue(item);
 		}
 		else {