Fix mypy errors with latest canonicaljson (#13905)

* Lockfile: update canonicaljson 1.6.0 -> 1.6.3

* Fix mypy errors with latest canonicaljson

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!

* Changelog

Author: David Robertson

changelog.d/13905.misc

@@ -0,0 +1 @@
+Fix mypy errors with canonicaljson 1.6.3.

poetry.lock

@@ -705,7 +705,7 @@ def stopProducing(self) -> None:
         self._request = None
 
 
-def _encode_json_bytes(json_object: Any) -> bytes:
+def _encode_json_bytes(json_object: object) -> bytes:
     """
     Encode an object into JSON. Returns an iterator of bytes.
     """
@@ -746,7 +746,7 @@ def respond_with_json(
         return None
 
     if canonical_json:
-        encoder = encode_canonical_json
+        encoder: Callable[[object], bytes] = encode_canonical_json
     else:
         encoder = _encode_json_bytes