Use optional type annotations

aleksis/apps/chronos/util/chronos_helpers.py

 from typing import Optional
 
-from django.contrib.auth.models import User
 from django.db.models import Count, Q
 from django.http import HttpRequest, HttpResponseNotFound
 from django.shortcuts import get_object_or_404
 
 from guardian.core import ObjectPermissionChecker
+from typing import TYPE_CHECKING
 
 from aleksis.core.models import Group, Person
 from aleksis.core.util.predicates import check_global_permission
@@ -13,6 +13,11 @@ from aleksis.core.util.predicates import check_global_permission
 from ..managers import TimetableType
 from ..models import LessonPeriod, LessonSubstitution, Room
 
+if TYPE_CHECKING:
+    from django.contrib.auth import get_user_model
+
+    User = get_user_model()  # noqa
+
 
 def get_el_by_pk(
     request: HttpRequest,
@@ -46,7 +51,7 @@ def get_substitution_by_id(request: HttpRequest, id_: int, week: int):
     ).first()
 
 
-def get_teachers(user: User):
+def get_teachers(user: "User"):
     """Get the teachers whose timetables are allowed to be seen by current user."""
     checker = ObjectPermissionChecker(user)
 
@@ -70,7 +75,7 @@ def get_teachers(user: User):
     return teachers
 
 
-def get_classes(user: User):
+def get_classes(user: "User"):
     """Get the classes whose timetables are allowed to be seen by current user."""
     checker = ObjectPermissionChecker(user)
 
@@ -104,7 +109,7 @@ def get_classes(user: User):
     return classes
 
 
-def get_rooms(user: User):
+def get_rooms(user: "User"):
     """Get the rooms whose timetables are allowed to be seen by current user."""
     checker = ObjectPermissionChecker(user)