Update helper script for to python3.
Update helper script for to python3.

apple/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf