fix(tests): fix fasting custom threshold tests using Django ORM cache

The user_with_custom_thresholds fixture used UserProfile.objects.get_or_create()
which returns a separate Python instance from the one cached in
user._state.fields_cache['profile']. When the fixture modified and saved
the get_or_create instance, the cached instance on the user object still
held default values (periodic=16, full=24). compute_fasting() accessed
u.profile which returned the stale cached object.

Fixed by modifying through u.profile directly, which operates on the
same cached instance. Also applied black formatting to fasting.py.

tests/trends_tests/test_fasting.py

@@ -2,7 +2,6 @@ from datetime import timedelta
 
 import time_machine
 from django.utils import timezone
-
 from drinks.models import Drink
 from lifeevents.models import LifeEvent
 from scrobbles.models import Scrobble