Spurred on by the note in #10734 about txn being redefined in for loop as we are iterating over it.

We were originally iterating over txn, which can work, but as txn was being used to run a
simple_delete operation inside the loop, it was actually being redefined mid-loop.

#10734

I was curious if there were other cases in the codebase, so I searched for in txn: and found one more case. It looks like this would cause a problem if there was an event where the event_auth_chains -> chain_id was not defined.

Linting

Maybe we should disallow iterating over txn directly in favor of using txn.fetchall()