linting fixes

- remove unused imports
- add class docstrings

bookwyrm/models/list.py

@@ -1,16 +1,15 @@
 """ make a list of books!! """
-from bookwyrm.models.group import GroupMember
-from dataclasses import field
 from django.apps import apps
 from django.db import models
 from django.utils import timezone
 
 from bookwyrm import activitypub
 from bookwyrm.settings import DOMAIN
+
 from .activitypub_mixin import CollectionItemMixin, OrderedCollectionMixin
 from .base_model import BookWyrmModel
-from . import fields
 from .group import GroupMember
+from . import fields
 
 CurationType = models.TextChoices(
     "Curation",