Resolved flake8 warnings

src/pretix/control/views/event.py

@@ -3,7 +3,7 @@ from django import forms
 
 from django.contrib import messages
 from django.db.models import Sum
-from django.forms import inlineformset_factory, formset_factory, modelformset_factory, BaseInlineFormSet
+from django.forms import modelformset_factory
 from django.shortcuts import render, redirect
 from django.utils.functional import cached_property
 from django.views.generic import FormView