update protobuf version

[NicolasM0]

google.protobuf.descriptor_pool don't have enum_types_by_name field in version 3.13.0

When importing mavsdk, I get:

  File "C:\Users\fclav\anaconda3\lib\site-packages\mavsdk\__init__.py", line 7, in <module>
    from .system import System
  File "C:\Users\fclav\anaconda3\lib\site-packages\mavsdk\system.py", line 8, in <module>
    from . import action
  File "C:\Users\fclav\anaconda3\lib\site-packages\mavsdk\action.py", line 5, in <module>
    from . import action_pb2, action_pb2_grpc
  File "C:\Users\fclav\anaconda3\lib\site-packages\mavsdk\action_pb2.py", line 16, in <module>
    from . import mavsdk_options_pb2 as mavsdk__options__pb2
  File "C:\Users\fclav\anaconda3\lib\site-packages\mavsdk\mavsdk_options_pb2.py", line 21, in <module>
    _ASYNCTYPE = DESCRIPTOR.enum_types_by_name['AsyncType']
AttributeError: 'NoneType' object has no attribute 'enum_types_by_name'

Updating protobuf to the last version (3.20.1) fix this issues

[JonasVautherin]

Thanks a lot! 🚀