Fix mypy errors with latest canonicaljson #13905

The change to `_encode_json_bytes` definition wasn't sufficient: ``` synapse/http/server.py:751: error: Incompatible types in assignment (expression has type "Callable[[Arg(object, 'json_object')], bytes]", variable has type "Callable[[Arg(object, 'data')], bytes]") [assignment] ``` Which I think is mypy warning us that the two functions accept different sets of kwargs. Fair enough!