Use consistent flake8 settings everywhere and fix flake8 issues

src/pretix/base/services/cart.py

@@ -8,7 +8,7 @@ from django.utils.translation import ugettext as _
 
 from pretix.base.i18n import LazyLocaleException
 from pretix.base.models import (
-    CartPosition, Event, EventLock, Item, ItemVariation, Quota, Voucher,
+    CartPosition, Event, Item, ItemVariation, Quota, Voucher,
 )
 from pretix.base.services.locking import LockTimeoutException
 from pretix.celery import app

src/pretix/base/services/export.py

@@ -1,6 +1,5 @@
 from typing import Any, Dict
 
-from django.conf import settings
 from django.core.files.base import ContentFile
 
 from pretix.base.models import CachedFile, Event, cachedfile_name

src/pretix/base/services/invoices.py

@@ -5,7 +5,6 @@ from datetime import date
 from decimal import Decimal
 from locale import format as lformat
 
-from django.conf import settings
 from django.contrib.staticfiles import finders
 from django.core.files.base import ContentFile
 from django.db import transaction

src/pretix/base/services/orders.py

@@ -15,7 +15,7 @@ from pretix.base.i18n import (
     LazyDate, LazyLocaleException, LazyNumber, language,
 )
 from pretix.base.models import (
-    CartPosition, Event, EventLock, Item, ItemVariation, Order, OrderPosition,
+    CartPosition, Event, Item, ItemVariation, Order, OrderPosition,
     Quota, User,
 )
 from pretix.base.models.orders import InvoiceAddress