Fix a few errors on preview billing rework

Another WIP commit to sync with laptop, do not forget to rebase!

uncloud/uncloud_pay/helpers.py

@@ -5,6 +5,8 @@ from rest_framework.viewsets import GenericViewSet
 from django.db.models import Q
 from .models import Bill, Payment, PaymentMethod, Order
 from django.utils import timezone
+from django.core.exceptions import ObjectDoesNotExist
+from dateutil.relativedelta import relativedelta
 
 def sum_amounts(entries):
     return reduce(lambda acc, entry: acc + entry.amount, entries, 0)