AttributeError: module 'json_msgs.msg' has no attribute 'Json'

Env:
Ubuntu 20.04 ROS-noetic

Hello,
I run the example code and I got the error below.
any help is appreciate, thanks

`publisher = rospy.Publisher(topicName,json_transport.PackedJson,queue_size=10)`
```
class PackedJson(json_msg.Json):
AttributeError: module 'json_msgs.msg' has no attribute 'Json'
```
