Use consistent flake8 settings everywhere and fix flake8 issues

src/pretix/base/views/cachedfiles.py

@@ -1,7 +1,7 @@
 import os
 
 from django.http import FileResponse, HttpRequest, HttpResponse
-from django.shortcuts import get_object_or_404, redirect
+from django.shortcuts import get_object_or_404
 from django.utils.functional import cached_property
 from django.views.generic import TemplateView