Discord approval views and the VoiceReceiver hold a reference to the
adapter's _allowed_user_ids set. Rebinding the attribute to a new set left
those holders authorizing a revoked user until restart; mutate the set in
place instead. list/str/None snapshots keep the rebinding behaviour.
The revoke test now pins that a held reference loses the revoked user,
and drops a tautological id(adapter) assertion plus an unused fixture.
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