Bumped notifications-utils to 3.7.0. Version 3.7.0 includes the
`convert_utc_to_bst` and `convert_bst_to_utc` functions and the
`LETTER_PROCESSING_DEADLINE` constant, so these have been removed from
this repo and anywhere using these has now been updated to get these
from `notifications-utils`.
Also bumped pytest by a patch version to bring in a bug fix.
it now uses the ft_notification_status table - nice and quick. if you
ask for the current tax year it'll top up the data with numbers from
the notification table.
the data is in exactly the same shape as the old endpoint - no changes
to the admin app are necessary