Remove Sphinx <9.0.0 pin

### Status [Upstream issue](https://github.com/executablebooks/sphinx-tabs/issues/206) has been fixed by https://github.com/executablebooks/sphinx-tabs/commit/b573a2f66a806a01adbb2141fdf498b070c52e2d, released in sphinx-tabs version 3.5.0. ### Description Tracking for comment in code: https://github.com/nextstrain/augur/blob/952612588eee847453a34a8401bca5914cbf69f8/setup.py#L89-L90