Reverse when step is negative

Author: HyeockJinKim

py/range.go

@@ -197,8 +197,14 @@ func computeRangeSlice(r *Range, s *Slice) (Object, error) {
 
 	var sliceLength Int
 	if startIndex < stopIndex {
+		if step < 0 {
+			startIndex, stopIndex = stopIndex-1, startIndex-1
+		}
 		sliceLength = (stopIndex - startIndex - 1) / stepIndex + 1
 	} else {
+		if step < 0 {
+			startIndex, stopIndex = stopIndex+1, startIndex+1
+		}
 		sliceLength = (stopIndex - startIndex + 1) / stepIndex + 1
 	}