calendar_utils: Fix tests

.isort.cfg

calendar_utils/models.py

calendar_utils/parser.py

tests/calendar_utils/conftest.py

tests/calendar_utils/sample.ics

tests/calendar_utils/samples/future_events.json

tests/calendar_utils/samples/past_events.json

tests/calendar_utils/samples/response.json

tests/calendar_utils/snapshots/__init__.py

tests/calendar_utils/snapshots/snap_test_models.py

tests/calendar_utils/snapshots/snap_test_parser.py

tests/calendar_utils/test_models.py

tests/calendar_utils/test_parser.py