Properly catch websocket CancelledError in websocket handler in Python 3.7

[ashleysommer]

Apply fix #2463 to Sanic 21.12LTS version for python 3.7 users.

[ashleysommer]

I'm guessing we don't need to worry about the linter checks failing on this?

[ahopkins]

That's odd... I wonder why that is? I am assuming it must be a versioning issue. But yes, I do not care about that.

[ashleysommer]

Black check failing:

...
would reformat sanic/config.py
...
would reformat sanic/headers.py
...

Maybe a newer version of black used now than in December release? Or maybe some patches were pushed to 21.12LTS branch previously that broke formatting.

[ahopkins]

I think it is a black version issue. I have run into this on some other projects.