Enhancements
* Add support for python 3.9
* Add support for pandas 1.2.0.
* Add support for [pandera schemas](https://pandera.readthedocs.io/en/stable/) via `SchemaSelector`.
Documentation Improvements
* Fix errors in examples.
* Clarify documentation structure
* Switch to [Furo](https://github.com/pradyunsg/furo) theme.
Testing improvements
* Increase coverage to 99%
Repo Improvements
* Simplify CI workflow