Sqlalchemy-celery-beat

- fix a bug where schedules with the same ID and different descriminators were selected when loading the `PeriodicTask` thanks to [Matthew Bronstein](https://github.com/mbronstein1) and [Bazyl Horsey](https://github.com/bazylhorsey)

- added tests
- fixed bug in `IntervalSchedule` repr function
- fixed bug in `PeriodicTaskChanged` insertion

- fixed bug where `one_off` is defaulting to `True`

- Add Validations to all models
- Use `Enum` instead of free text for `IntervalSchedule.period` and `SolarSchedule.event`
- Use generic foreign key for schedules in `PeriodicTask`
- Make sessions efficient and only use a session when needed
- Allow using a separate schema for the tables
- Add `ClockedSchedule`
- Improve timezone support

- Support Celery 5.0.1