Update tokio requirement from 0.2 to 1.7

[dependabot-preview[bot]]

Updates the requirements on tokio to permit the latest version.

Release notes

Sourced from tokio's releases.

Tokio v1.7.0

1.7.0 (June 15, 2021)

Added

• net: add named pipes on windows (#3760)
• net: add TcpSocket from std::net::TcpStream conversion (#3838)
• sync: add receiver_count to watch::Sender (#3729)
• sync: export sync::notify::Notified future publicly (#3840)
• tracing: instrument task wakers (#3836)

Fixed

• macros: suppress clippy::default_numeric_fallback lint in generated code (#3831)
• runtime: immediately drop new tasks when runtime is shut down (#3752)
• sync: deprecate unused mpsc::RecvError type (#3833)

Documented

• io: clarify EOF condition for AsyncReadExt::read_buf (#3850)
• io: clarify limits on return values of AsyncWrite::poll_write (#3820)
• sync: add examples to Semaphore (#3808)

Commits

• 34c6a26 chore: prepare Tokio v1.7.0 (#3863)
• 97e7830 net: provide NamedPipe{Client, Server} types and builders (#3760)
• 606206e Merge branch 'tokio-1.6.x' into merge-1.6.x
• dfe4013 chore: prepare Tokio 1.6.2 release (#3859)
• 18779aa time: fix time::advance() with sub-ms durations (#3852)
• 2c24a02 chore: re-enable test_socket_pair on FreeBSD (#3844)
• f55b77a tracing: emit waker op as str instead as Debug (#3853)
• 21de476 sync: export sync::notify::Notified future publicly (#3840)
• cb147a2 sync: deprecate mpsc::RecvError (#3833)
• f759240 chore: update version in README.md (#3851)
• Additional commits viewable in compare view

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[dependabot-preview[bot]]

Superseded by #95.