Refactor out function to get page count

This has been moved to the letters utils file since it will be used in
more than one place. The notification parameter has been removed so that
the function can be used when we don't have a notification id.

app/celery/letters_pdf_tasks.py

@@ -1,4 +1,3 @@
-import io
 import math
 import base64
 from datetime import datetime
@@ -9,7 +8,6 @@ from base64 import urlsafe_b64encode
 from PyPDF2.utils import PdfReadError
 from botocore.exceptions import ClientError as BotoClientError
 from flask import current_app
-from notifications_utils.pdf import pdf_page_count
 from requests import (
     post as requests_post,
     RequestException
@@ -39,7 +37,8 @@ from app.letters.utils import (
     move_failed_pdf,
     move_scan_to_invalid_pdf_bucket,
     move_error_pdf_to_scan_bucket,
-    get_file_names_from_error_bucket
+    get_file_names_from_error_bucket,
+    get_page_count,
 )
 from app.models import (
     KEY_TYPE_TEST,
@@ -211,8 +210,9 @@ def process_virus_scan_passed(self, filename):
     old_pdf = scan_pdf_object.get()['Body'].read()
 
     try:
-        billable_units = _get_page_count(notification, old_pdf)
+        billable_units = get_page_count(old_pdf)
     except PdfReadError:
+        current_app.logger.exception(msg='Invalid PDF received for notification_id: {}'.format(notification.id))
         _move_invalid_letter_and_update_status(notification, filename, scan_pdf_object)
         return
 
@@ -267,17 +267,6 @@ def process_virus_scan_passed(self, filename):
         update_notification_status_by_id(notification.id, NOTIFICATION_TECHNICAL_FAILURE)
 
 
-def _get_page_count(notification, old_pdf):
-    try:
-        pages = pdf_page_count(io.BytesIO(old_pdf))
-        pages_per_sheet = 2
-        billable_units = math.ceil(pages / pages_per_sheet)
-        return billable_units
-    except PdfReadError as e:
-        current_app.logger.exception(msg='Invalid PDF received for notification_id: {}'.format(notification.id))
-        raise e
-
-
 def _move_invalid_letter_and_update_status(notification, filename, scan_pdf_object):
     try:
         move_scan_to_invalid_pdf_bucket(filename)