SF bug 994255: Py_RETURN_NONE causes too much warnings

Rewrote Py_RETURN_{NONE, TRUE, FALSE} to expand to comma expressions
rather than "do {} while(0)" thingies.  The OP complained because he
likes using MS /W4 sometimes, and then all his uses of these things
generate nuisance warnings about testing a constant expression (in
the "while(0)" part).  Comma expressions don't have this problem
(although it's a lucky accident that comma expressions suffice for these
macros!).
diff --git a/Include/object.h b/Include/object.h
index 60cf146..5db4dac 100644
--- a/Include/object.h
+++ b/Include/object.h
@@ -650,7 +650,7 @@
 #define Py_None (&_Py_NoneStruct)
 
 /* Macro for returning Py_None from a function */
-#define Py_RETURN_NONE do {Py_INCREF(Py_None); return Py_None;} while (0)
+#define Py_RETURN_NONE return Py_INCREF(Py_None), Py_None
 
 /*
 Py_NotImplemented is a singleton used to signal that an operation is