Update sphinx requirement from ~=2.0.1 to ~=2.1.0
Update sphinx requirement from ~=2.0.1 to ~=2.1.0

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](https://github.com/sphinx-doc/sphinx/compare/v2.0.1...v2.1.0)
ariebovenberg/snug,ariebovenberg/omgorm