Bump sphinx from 3.4.3 to 3.5.1 in /ManualGenerator
Bump sphinx from 3.4.3 to 3.5.1 in /ManualGenerator

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.4.3 to 3.5.1.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](https://github.com/sphinx-doc/sphinx/compare/v3.4.3...v3.5.1)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
y-iihoshi/ThScoreFileConverter,y-iihoshi/ThScoreFileConverter