build(deps-dev): bump eslint from 6.8.0 to 7.21.0 (#185)

Bumps [eslint](https://github.com/eslint/eslint) from 6.8.0 to 7.21.0.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](eslint/eslint@v6.8.0...v7.21.0)

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Author: dependabot[bot]