Fix issues reported by pylama.

biscuit/apps/chronos/models.py

 from datetime import datetime
-from typing import Dict, Optional, List, Tuple
+from typing import Dict, Optional, Tuple
 
 from django.core import validators
 from django.db import models
@@ -7,8 +7,6 @@ from django.utils.translation import ugettext_lazy as _
 
 from .util import current_week
 
-from biscuit.core.models import Person
-
 
 class TimePeriod(models.Model):
     WEEKDAY_CHOICES = [

biscuit/apps/chronos/views.py

@@ -11,10 +11,10 @@ from django.utils.translation import ugettext as _
 from django_tables2 import RequestConfig
 
 from biscuit.core.decorators import admin_required
-from biscuit.core.models import Group, Person
+from biscuit.core.util import messages
 
 from .forms import SelectForm, LessonSubstitutionForm
-from .models import LessonPeriod, TimePeriod, Room, LessonSubstitution
+from .models import LessonPeriod, TimePeriod, LessonSubstitution
 from .util import current_week, week_weekday_from_date, week_days
 from .tables import LessonsTable