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/config.py

@@ -1,4 +1,4 @@
-from datetime import timedelta, time
+from datetime import timedelta
 import os
 import json
 
@@ -318,8 +318,6 @@ class Config(object):
     DOCUMENT_DOWNLOAD_API_HOST = os.environ.get('DOCUMENT_DOWNLOAD_API_HOST', 'http://localhost:7000')
     DOCUMENT_DOWNLOAD_API_KEY = os.environ.get('DOCUMENT_DOWNLOAD_API_KEY', 'auth-token')
 
-    LETTER_PROCESSING_DEADLINE = time(17, 30)
-
     MMG_URL = "https://api.mmg.co.uk/json/api.php"
     AWS_REGION = 'eu-west-1'