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/notifications/process_notifications.py

@@ -9,6 +9,7 @@ from notifications_utils.recipients import (
     validate_and_format_phone_number,
     format_email_address
 )
+from notifications_utils.timezones import convert_bst_to_utc, convert_utc_to_bst
 
 from app import redis_store
 from app.celery import provider_tasks
@@ -34,8 +35,6 @@ from app.v2.errors import BadRequestError
 from app.utils import (
     cache_key_for_service_template_counter,
     cache_key_for_service_template_usage_per_day,
-    convert_bst_to_utc,
-    convert_utc_to_bst,
     get_template_instance,
 )