Update tokio requirement from 0.2 to 1.0

[dependabot[bot]]

Updates the requirements on tokio to permit the latest version.

Release notes

Sourced from tokio's releases.

Tokio v1.0.0

Commit to the API and long-term support.

Announcement and more details.

Fixed

• sync: spurious wakeup in watch (#3234).

Changed

• io: rename AsyncFd::with_io() to try_io() (#3306)
• fs: avoid OS specific *Ext traits in favor of conditionally defining the fn (#3264).
• fs: Sleep is !Unpin (#3278).
• net: pass SocketAddr by value (#3125).
• net: TcpStream::poll_peek takes ReadBuf (#3259).
• rt: rename runtime::Builder::max_threads() to max_blocking_threads() (#3287).
• time: require current_thread runtime when calling time::pause() (#3289).

Removed

• remove tokio::prelude (#3299).
• io: remove AsyncFd::with_poll() (#3306).
• net: remove {Tcp,Unix}Stream::shutdown() in favor of AsyncWrite::shutdown() (#3298).
• stream: move all stream utilities to tokio-stream until Stream is added to std (#3277).
• sync: mpsc try_recv() due to unexpected behavior (#3263).
• tracing: make unstable as tracing-core is not 1.0 yet (#3266).

Added

• fs: poll_* fns to DirEntry (#3308).
• io: poll_* fns to io::Lines, io::Split (#3308).
• io: _mut method variants to AsyncFd (#3304).
• net: poll_* fns to UnixDatagram (#3223).
• net: UnixStream readiness and non-blocking ops (#3246).
• sync: UnboundedReceiver::blocking_recv() (#3262).
• sync: watch::Sender::borrow() (#3269).
• sync: Semaphore::close() (#3065).
• sync: poll_recv fns to mpsc::Receiver, mpsc::UnboundedReceiver (#3308).
• time: poll_tick fn to time::Interval (#3316).

Commits

• a66017f chore: prepare Tokio 1.0 release (#3319)
• aa6597b compat: update traits naming to match tokio 1.0 (#3324)
• ce0e9c6 chore: Revert "use #[non_exhaustive] instead of private unit field" (#3323)
• 575938d chore: use #[non_exhaustive] instead of private unit field (#3320)
• 0deaeb8 chore: remove unused slab dependency (#3318)
• a8dda19 chore: update to released bytes 1.0 (#3317)
• be9fdb6 time: make Interval::poll_tick() public (#3316)
• 2ee9520 fs: misc small API tweaks (#3315)
• 7d28e4c examples: add futures executor threadpool (#3198)
• 0b83b3b fs,sync: expose poll_ fns on misc types (#3308)
• Additional commits viewable in compare view

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[dependabot[bot]]

Looks like tokio is no longer updatable, so this is no longer needed.