Bump futures from 0.3.5 to 0.3.7 (mozilla#304)

Bumps [futures](https://github.com/rust-lang/futures-rs) from 0.3.5 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@0.3.5...0.3.7)

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