linting fixes

- remove unused imports
- add class docstrings

bookwyrm/models/group.py

@@ -1,6 +1,6 @@
 """ do book related things with other users """
 from django.apps import apps
-from django.db import models, IntegrityError, models, transaction
+from django.db import models, IntegrityError, transaction
 from django.db.models import Q
 from bookwyrm.settings import DOMAIN
 from .base_model import BookWyrmModel
@@ -36,6 +36,7 @@ class GroupMember(models.Model):
     )
 
     class Meta:
+        """Users can only have one membership per group"""
         constraints = [
             models.UniqueConstraint(fields=["group", "user"], name="unique_membership")
         ]
@@ -83,6 +84,7 @@ class GroupMemberInvitation(models.Model):
     )
 
     class Meta:
+        """Users can only have one outstanding invitation per group"""
         constraints = [
             models.UniqueConstraint(fields=["group", "user"], name="unique_invitation")
         ]