A newsletter keeps every sign-up it has ever received, as a list of dictionaries with the keys "email" and "town". When someone moves, they sign up again with their new town, so some emails appear more than once.
Each email should appear once, with the town from its latest sign-up. The list is in time order, so later in the list means more recent.
Write latest_towns(signups). Return a list of [email, town] pairs, one per email, in the order the kept rows sit in the original list.
Emails count as the same only if they match exactly. signups always has at least one row.