Fix Pydantic deprecation and add mypy __all__ exports

[abrookins]

Fixes #728 and #468.

Changes

1. Fix Pydantic V3 deprecation warning - Changed self.model_fields to self.__class__.model_fields in _collect_expirations() to avoid the deprecation warning about accessing model_fields from an instance.

2. Add __all__ export list for mypy compatibility - User code with from aredis_om import ... now passes mypy --strict without errors about missing explicit exports.