Issue 1820:  structseq objects did not work with the % formatting operator or isinstance(t, tuple).

Orignal patch (without tests) by Leif Walsh.
diff --git a/Objects/structseq.c b/Objects/structseq.c
index b6126ba..6b1b590 100644
--- a/Objects/structseq.c
+++ b/Objects/structseq.c
@@ -460,7 +460,7 @@
 	structseq_methods,      		/* tp_methods */
         NULL,			             	/* tp_members */
 	0,			          	/* tp_getset */
-	0,					/* tp_base */
+	&PyTuple_Type,				/* tp_base */
 	0,					/* tp_dict */
 	0,					/* tp_descr_get */
 	0,					/* tp_descr_set */