Bump notifications-utils to 3.7.0

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.

app/dao/date_util.py

@@ -1,9 +1,8 @@
 from datetime import datetime, timedelta
 
+from notifications_utils.timezones import convert_bst_to_utc
 import pytz
 
-from app.utils import convert_bst_to_utc
-
 
 def get_months_for_financial_year(year):
     return [

app/dao/fact_billing_dao.py

@@ -1,6 +1,7 @@
 from datetime import datetime, timedelta, time
 
 from flask import current_app
+from notifications_utils.timezones import convert_bst_to_utc, convert_utc_to_bst
 from sqlalchemy.dialects.postgresql import insert
 from sqlalchemy import func, case, desc, Date, Integer
 
@@ -20,7 +21,6 @@ from app.models import (
     EMAIL_TYPE,
     NOTIFICATION_STATUS_TYPES_BILLABLE_FOR_LETTERS
 )
-from app.utils import convert_utc_to_bst, convert_bst_to_utc
 
 
 def fetch_billing_totals_for_year(service_id, year):

app/dao/fact_notification_status_dao.py

@@ -1,6 +1,7 @@
 from datetime import datetime, timedelta, time
 
 from flask import current_app
+from notifications_utils.timezones import convert_bst_to_utc
 from sqlalchemy import func
 from sqlalchemy.dialects.postgresql import insert
 from sqlalchemy.sql.expression import literal
@@ -8,7 +9,7 @@ from sqlalchemy.types import DateTime, Integer
 
 from app import db
 from app.models import Notification, NotificationHistory, FactNotificationStatus, KEY_TYPE_TEST
-from app.utils import convert_bst_to_utc, get_london_midnight_in_utc, midnight_n_days_ago
+from app.utils import get_london_midnight_in_utc, midnight_n_days_ago
 
 
 def fetch_notification_status_for_day(process_day, service_id=None):

app/dao/notifications_dao.py

@@ -20,6 +20,7 @@ from sqlalchemy.orm import joinedload
 from sqlalchemy.sql.expression import case
 from sqlalchemy.sql import functions
 from notifications_utils.international_billing_rates import INTERNATIONAL_BILLING_RATES
+from notifications_utils.timezones import convert_utc_to_bst
 
 from app import db, create_uuid
 from app.aws.s3 import remove_s3_object, get_s3_bucket_objects
@@ -48,7 +49,7 @@ from app.models import (
 )
 
 from app.dao.dao_utils import transactional
-from app.utils import convert_utc_to_bst, get_london_midnight_in_utc
+from app.utils import get_london_midnight_in_utc
 
 
 @statsd(namespace="dao")