Get rid of warnings on some platforms by using %u for a size_t.

Objects/stringobject.c

@@ -274,7 +274,7 @@ PyString_FromFormatV(const char *format, va_list vargs)
 				if (longflag)
 					sprintf(s, "%ld", va_arg(vargs, long));
 				else if (size_tflag)
-					sprintf(s, "%" PY_FORMAT_SIZE_T "d",
+					sprintf(s, "%" PY_FORMAT_SIZE_T "u",
 					        va_arg(vargs, size_t));
 				else
 					sprintf(s, "%d", va_arg(vargs, int));