Use `%r` rather than `%s` for stringifying events (#8679)

otherwise non-state events get written as `<FrozenEvent ... state_key='None'>`
which is indistinguishable from state events with the actual state_key `None`.

changelog.d/8679.misc

+Clarify representation of events in logfiles.

synapse/events/__init__.py

@@ -368,7 +368,7 @@ class FrozenEvent(EventBase):
         return self.__repr__()
 
     def __repr__(self):
-        return "<FrozenEvent event_id='%s', type='%s', state_key='%s'>" % (
+        return "<FrozenEvent event_id=%r, type=%r, state_key=%r>" % (
             self.get("event_id", None),
             self.get("type", None),
             self.get("state_key", None),
@@ -451,7 +451,7 @@ class FrozenEventV2(EventBase):
         return self.__repr__()
 
     def __repr__(self):
-        return "<%s event_id='%s', type='%s', state_key='%s'>" % (
+        return "<%s event_id=%r, type=%r, state_key=%r>" % (
             self.__class__.__name__,
             self.event_id,
             self.get("type", None),