Fix overly restrictive typeguards in get_rooms_that_allow_join

[blackmad]

This was causing a bug that was only occuring when the server had modules loaded and there were unjoined space-visible rooms in a hierarchy.

This fix is a result of the following investigation:

Symptom: new 'space-visible' rooms are not showing up in /hierarchy Reason: because get_rooms_that_allow_join is checking that the allow key is a list, and it's coming back as a tuple synapse/handlers/event_auth.py#L329

Reason: when the room is created, the join_rules event creation logic calls check_event_allowed - synapse/handlers/message.py#L1289 - which calls event.freeze() (in synapse/module_api/callbacks/third_party_event_rules_callbacks.py#L294 ) which seems to be changing lists to tuples - so that when the event is being pulled from cache, the allow list is a tuple. But when I restart the server, it gets pulled from the DB and deserialized as a list.

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