(Merge into trunk.)

Fix for SF bug #492345.  (I could've sworn I checked this in, but
apparently I didn't!)

This code:

    class Classic:
        pass

    class New(Classic):
        __metaclass__ = type

attempts to create a new-style class with only classic bases -- but it
doesn't work right.  Attempts to fix it so it works caused problems
elsewhere, so I'm now raising a TypeError in this case.

Objects/typeobject.c

@@ -757,7 +757,9 @@ best_base(PyObject *bases)
 			return NULL;
 		}
 	}
-	assert(base != NULL);
+	if (base == NULL)
+		PyErr_SetString(PyExc_TypeError,
+			"a new-style class can't have only classic bases");
 	return base;
 }