Update to SQLAlchemy 2.0

[Daniel-Faber]

The used sqlalchemy.orm.mappers are deprecated in SQLAlchemy 2.0, alongside other breaking changes fixed in this PR. Mainly we have to use sqlalchemy.orm.registry.map_imperatively instead of sqlalchemy.orm.mappers. We can also re-use the mapper_registry to replace sqlalchemy.Metadata, see the docs: https://docs.sqlalchemy.org/en/20/orm/mapping_styles.html#imperative-mapping

Full Changelog fixing all breaking changes in SQLAlchemy 2.0:

• Use sqlalchemy.sql.text when running sqlalchemy.orm.session.execute
• Convert rows to dicts via row._asdict() instead of dict(r)
• Use sqlalchemy.orm.registry.map_imperatively instead of sqlalchemy.orm.mappers
• Optional: Use sqlalchemy.orm.registry.metadata instead of sqlalchemy.Metadata