Use consistent flake8 settings everywhere and fix flake8 issues

src/pretix/base/models/invoices.py

@@ -3,7 +3,6 @@ from datetime import date
 from decimal import Decimal
 
 from django.db import DatabaseError, models, transaction
-from django.db.models import Max
 from django.utils.crypto import get_random_string
 from django.utils.functional import cached_property
 

src/pretix/base/models/vouchers.py

@@ -7,7 +7,7 @@ from django.utils.translation import ugettext_lazy as _
 from .base import LoggedModel
 from .event import Event
 from .items import Item, ItemVariation, Quota
-from .orders import CartPosition, Order, OrderPosition
+from .orders import CartPosition
 
 
 def _generate_random_code():