Time-Piece: Use isUPPER, not isupper

The latter gives results that are dependent on the program's underlying
locale, and so may be inconsistent.

If locale dependence is actually desired, isUPPER_LC should be used, as
it knows about various things the module writer shouldn't have to
concern themselves with.  It is supported since 5.004

cpan/Time-Piece/Piece.xs

@@ -848,7 +848,7 @@ label:
 			const char *cp;
 			char *zonestr;
 
-			for (cp = buf; *cp && isupper((unsigned char)*cp); ++cp)
+			for (cp = buf; *cp && isUPPER((unsigned char)*cp); ++cp)
                             {/*empty*/}
 			if (cp - buf) {
 				zonestr = (char *)malloc((size_t) (cp - buf + 1));