Add typing information

[palfrey]

Summary:

• Celery Version: 4.4.7
• Celery-Beat Version: 2.1.0

Exact steps to reproduce the issue:

1. Run mypy against an app with this library.
2. Get error like error: Skipping analyzing 'django_celery_beat.models': found module but no type hints or library stubs

[auvipy]

we will gradually do this in future. but currently only py-amqp, kombu & celery is main target for type hinting