Import 'RobotStateClient' failure for go2

[singleye]

I'm trying to use RobotStateClient to check the service status and found this issue, it's because it tried to import a non-exist module.

>>> from unitree_sdk2py.go2.robot_state.robot_state_client import RobotStateClient                                                                                                            
Traceback (most recent call last):                                                                                                                                                            
  File "<stdin>", line 1, in <module>                                                                                                                                                         
  File ".........../unitree_sdk2_python/unitree_sdk2py/go2/robot_state/robot_state_client.py", line 4, in <module>                     
    from ...rpc.client_internal import *                                                                                                                                                      
ModuleNotFoundError: No module named 'unitree_sdk2py.rpc.client_internal'    

Code commit: 0d86c54

I create a merge request to try to fix it, it seems work.

#30