Properly check for frozendicts in event auth code. (matrix-org#14864)

Check for for an instance of a mapping instead of a dict.

This only affects room version 10 when frozen events are enabled.

changelog.d/14864.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in Synapse 1.64.0 when using room version 10 with frozen events enabled.

synapse/event_auth.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import collections.abc
 import logging
 import typing
 from typing import (
@@ -877,7 +878,7 @@ def _check_power_levels(
                 if not isinstance(v, int):
                     raise SynapseError(400, f"{v!r} must be an integer.")
             if k in {"events", "notifications", "users"}:
-                if not isinstance(v, dict) or not all(
+                if not isinstance(v, collections.abc.Mapping) or not all(
                     isinstance(v, int) for v in v.values()
                 ):
                     raise SynapseError(