SF bug 994255: Py_RETURN_NONE causes too much warnings

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!).