Use CalendarWeek from calendarweek.

aleksis/apps/alsijil/model_extensions.py

 from datetime import date
+from calendarweek import CalendarWeek
 
 from django.db.models import Exists, F, OuterRef
 
 from aleksis.apps.chronos.models import LessonPeriod
-from aleksis.apps.chronos.util import CalendarWeek
 from aleksis.core.models import Group, Person
 
 from .models import PersonalNote

aleksis/apps/alsijil/views.py

+from calendarweek import CalendarWeek
 from datetime import date, datetime, timedelta
 from typing import Optional
 
@@ -12,7 +13,6 @@ from django.utils.translation import ugettext as _
 from django_tables2 import RequestConfig
 
 from aleksis.apps.chronos.models import LessonPeriod
-from aleksis.apps.chronos.util import CalendarWeek
 from aleksis.core.decorators import admin_required
 from aleksis.core.models import Group, Person, School
 from aleksis.core.util import messages