Use immutabledict instead of frozendict (#15113)

Additionally:

* Consistently use `freeze()` in test

---------

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: 6543 <6543@obermui.de>

1 files changed, 2 insertions, 2 deletions

diff --git a/synapse/events/snapshot.py b/synapse/events/snapshot.py

index c04ad08cbb..9b4d692cf4 100644
--- a/synapse/events/snapshot.py
+++ b/synapse/events/snapshot.py
@@ -15,7 +15,7 @@ from abc import ABC, abstractmethod
 from typing import TYPE_CHECKING, List, Optional, Tuple
 
 import attr
-from frozendict import frozendict
+from immutabledict import immutabledict
 
 from synapse.appservice import ApplicationService
 from synapse.events import EventBase
@@ -489,4 +489,4 @@ def _decode_state_dict(
     if input is None:
         return None
 
-    return frozendict({(etype, state_key): v for etype, state_key, v in input})
+    return immutabledict({(etype, state_key): v for etype, state_key, v in input})