fixup! Improve hints for ManyToManyDescriptor and start using it

Author: flaeppe

scripts/stubtest/allowlist_todo.txt

@@ -168,7 +168,6 @@ django.contrib.auth.models.Group.name
 django.contrib.auth.models.Group.permissions
 django.contrib.auth.models.Group.user_set
 django.contrib.auth.models.GroupManager.__slotnames__
-django.contrib.auth.models.Group_RelatedManager
 django.contrib.auth.models.Permission.codename
 django.contrib.auth.models.Permission.content_type
 django.contrib.auth.models.Permission.content_type_id
@@ -198,7 +197,6 @@ django.contrib.auth.models.User.password
 django.contrib.auth.models.User.user_permissions
 django.contrib.auth.models.User.username
 django.contrib.auth.models.UserManager.__slotnames__
-django.contrib.auth.models.User_RelatedManager
 django.contrib.auth.password_validation.CommonPasswordValidator.DEFAULT_PASSWORD_LIST_PATH
 django.contrib.auth.password_validation.CommonPasswordValidator.__init__
 django.contrib.auth.password_validation.PasswordValidator