Fix PushRuleEvaluator and Filter to work on frozendicts (#12100)

* Fix `PushRuleEvaluator` to work on frozendicts

frozendicts do not (necessarily) inherit from dict, so this needs to handle
them correctly.

* Fix event filtering for frozen events

Looks like this one was introduced by #11194.

tests/push/test_push_rule_evaluator.py

@@ -14,6 +14,8 @@
 
 from typing import Any, Dict
 
+import frozendict
+
 from synapse.api.room_versions import RoomVersions
 from synapse.events import FrozenEvent
 from synapse.push import push_rule_evaluator
@@ -191,6 +193,13 @@ class PushRuleEvaluatorTestCase(unittest.TestCase):
             "pattern should only match at the start/end of the value",
         )
 
+        # it should work on frozendicts too
+        self._assert_matches(
+            condition,
+            frozendict.frozendict({"value": "FoobaZ"}),
+            "patterns should match on frozendicts",
+        )
+
         # wildcards should match
         condition = {
             "kind": "event_match",