Use new canonicaljson serisalistion hook

From matrix-org/python-canonicaljson#59

synapse/__init__.py

@@ -20,6 +20,7 @@
 import json
 import os
 import sys
+from typing import Any, Dict
 
 from synapse.util.rust import check_rust_lib_up_to_date
 from synapse.util.stringutils import strtobool
@@ -63,9 +64,15 @@
 
 # Use the standard library json implementation instead of simplejson.
 try:
-    from canonicaljson import set_json_library
+    from canonicaljson import register_preserialisation_callback, set_json_library
+    from immutabledict import immutabledict
 
     set_json_library(json)
+
+    def _immutabledict_cb(d: immutabledict) -> Dict[str, Any]:
+        return d._dict
+
+    register_preserialisation_callback(immutabledict, _immutabledict_cb)
 except ImportError:
     pass