Update utility code to handle C implementations of frozendict (#10902)

* update _handle_frozendict to work with c implementations of frozen dict

* add changelog

* add clarifying comment to _handle_frozendict

changelog.d/10902.misc

@@ -0,0 +1 @@
+Update utility code to handle C implementations of frozendict.

synapse/util/__init__.py

@@ -50,7 +50,13 @@ def _handle_frozendict(obj: Any) -> Dict[Any, Any]:
     if type(obj) is frozendict:
         # fishing the protected dict out of the object is a bit nasty,
         # but we don't really want the overhead of copying the dict.
-        return obj._dict
+        try:
+            return obj._dict
+        except AttributeError:
+            # When the C implementation of frozendict is used,
+            # there isn't a `_dict` attribute with a dict
+            # so we resort to making a copy of the frozendict
+            return dict(obj)
     raise TypeError(
         "Object of type %s is not JSON serializable" % obj.__class__.__name__
     )