Bumps [attrs](https://github.com/python-attrs/attrs) from 20.2.0 to 20.3.0.
- [Release notes](https://github.com/python-attrs/attrs/releases)
- [Changelog](https://github.com/python-attrs/attrs/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/python-attrs/attrs/compare/20.2.0...20.3.0)