Make event objects unhashable

Generally speaking one should look up an event by its id. Otherwise
we'll fall back to `object.__hash__` (the instance's address, under
CPython) which is probably not what you want.

Noticed in #15240.

synapse/events/__init__.py

@@ -24,6 +24,7 @@
     Generic,
     Iterable,
     List,
+    NoReturn,
     Optional,
     Sequence,
     Tuple,
@@ -338,6 +339,9 @@ def __init__(
     type: DictProperty[str] = DictProperty("type")
     user_id: DictProperty[str] = DictProperty("sender")
 
+    def __hash__(self) -> NoReturn:
+        raise NotImplementedError()
+
     @property
     def event_id(self) -> str:
         raise NotImplementedError()