Bump mio from 1.0.3 to 1.0.4

Bumps [mio](https://github.com/tokio-rs/mio) from 1.0.3 to 1.0.4.
- [Release notes](https://github.com/tokio-rs/mio/releases)
- [Changelog](https://github.com/tokio-rs/mio/blob/master/CHANGELOG.md)
- [Commits](https://github.com/tokio-rs/mio/commits)

---
updated-dependencies:
- dependency-name: mio
  dependency-version: 1.0.4
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Author: dependabot[bot]