Bump structopt from 0.3.20 to 0.3.21 (#2046)

Bumps [structopt](https://github.com/TeXitoi/structopt) from 0.3.20 to 0.3.21.
- [Release notes](https://github.com/TeXitoi/structopt/releases)
- [Changelog](https://github.com/TeXitoi/structopt/blob/master/CHANGELOG.md)
- [Commits](https://github.com/TeXitoi/structopt/compare/v0.3.20...v0.3.21)

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