Hi there,
so yesterday I tried to find a combination of sanic 21.6.0, sanic-cors, sanic-routing ans sanic-plugins-framework, which was also compatible with rasa 3.0.4 - well, there was none. So I decided to try even though there was sanic-plugins-framework 0.95, which was not compatible with the rest… 
good news - this error message was gone, but bad news - it didn’t work
So today I decided I would create a new conda environment and installed:
rasa 3.0.0; rasa-sdk 3.0.0; sanic 21.6.0; sanic-cors 1.0.0: sanic-routing 0.7.0; nodejs v17.4.0; npm 8.3.1; socket.io 4.4.1
Basically, I tried what you suggestes here. And it is working like a charm. 
Do I need to be concerned that rasa 3.0.0 is not compatible with sanic-routung 0.7.0?
Thanks again for your help!