Suppose `change is MatchChange.no_change`. Then we will return on 332,
and hit neither the now_true branch nor the `else` branch. Therefore the
`if` can be turned into an `elif` with no behaviour change.

Justification:

- the only way to hit the final else branch is if `change is
  MatchChange.now_false`
- if `not is_in_room`, then the `state_key` will be
  one of the `user_id in user_ids` and so we'll call
  `self._handle_remove_user` with the `state_key` and return early
- therefore the call to `_handle_remove_user` in the final else branch
  was only hit if `is_in_room`, and so it can be moved to first `else`
  branch.

Having made this change we now have three mutually exclusive
cases (now_false, now_true, no_change). Therefore I can change if->elif
for the no_change branch.