chore(deps): bump futures-util from 0.3.6 to 0.3.7 (#30)

Bumps [futures-util](https://github.com/rust-lang/futures-rs) from 0.3.6 to 0.3.7.
- [Release notes](https://github.com/rust-lang/futures-rs/releases)
- [Changelog](https://github.com/rust-lang/futures-rs/blob/master/CHANGELOG.md)
- [Commits](rust-lang/futures-rs@0.3.6...0.3.7)

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