mapper() func is deprectiated

Per the SQLAlchemy docs, [the `mapper()` function is depreciated](https://docs.sqlalchemy.org/en/14/orm/mapping_styles.html#imperative-a-k-a-classical-mappings). The docs suggest using `registry.map_imperatively()` to get the same functionality used in pysheeet.

 @crazyguitar I'd be happy to open a PR & update the syntax - just be patient with me, as I'm still learning :)