Bump six from 1.15.0 to 1.16.0

Bumps [six](https://github.com/benjaminp/six) from 1.15.0 to 1.16.0.
- [Release notes](https://github.com/benjaminp/six/releases)
- [Changelog](https://github.com/benjaminp/six/blob/master/CHANGES)
- [Commits](https://github.com/benjaminp/six/compare/1.15.0...1.16.0)