recursive_isinstance(), recursive_issubclass():  New code here returned
NULL in case of error, but the functions are declared to return int.
MSVC 6 properly complains about that.  Return -1 on error instead.
diff --git a/Objects/abstract.c b/Objects/abstract.c
index 3d6d829..bf60c75 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -2033,7 +2033,7 @@
                 if (!recursion_depth) {
                     PyErr_SetString(PyExc_RuntimeError,
                                     "nest level of tuple too deep");
-                    return NULL;
+                    return -1;
                 }
 
 		n = PyTuple_GET_SIZE(cls);
@@ -2088,7 +2088,7 @@
                         if (!recursion_depth) {
                             PyErr_SetString(PyExc_RuntimeError,
                                             "nest level of tuple too deep");
-                            return NULL;
+                            return -1;
                         }
 			for (i = 0; i < n; ++i) {
 				retval = recursive_issubclass(