Update tokio requirement from 0.3 to 1.3

[dependabot[bot]]

Updates the requirements on tokio to permit the latest version.

Release notes

Sourced from tokio's releases.

Tokio v1.3.0

Added

• coop: expose an unconstrained() opt-out (#3547)
• net: add into_std for net types without it (#3509)
• sync: add same_channel method to mpsc::Sender (#3532)
• sync: add {try_,}acquire_many_owned to Semaphore (#3535)
• sync: add back RwLockWriteGuard::map and RwLockWriteGuard::try_map (#3348)

Fixed

• sync: allow oneshot::Receiver::close after successful try_recv (#3552)
• time: do not panic on timeout(Duration::MAX) (#3551)

Documented

• doc: doc aliases for pre-1.0 function names (#3523)
• io: fix typos (#3541)
• io: note the EOF behaviour of read_until (#3536)
• io: update AsyncRead::poll_read doc (#3557)
• net: update UdpSocket splitting doc (#3517)
• runtime: add link to LocalSet on new_current_thread (#3508)
• runtime: update documentation of thread limits (#3527)
• sync: do not recommend join_all for Barrier (#3514)
• sync: documentation for oneshot (#3592)
• sync: rename notify to notify_one (#3526)
• time: fix typo in Sleep doc (#3515)
• time: sync interval.rs and time/mod.rs docs (#3533)

Commits

• d0e4dd1 chore: prepare Tokio v1.3.0 (#3597)
• 872bc09 examples: update chat example (#3587)
• db1d904 util: fuse PollSemaphore (#3578)
• 05eeea5 coop: expose an unconstrained() opt-out (#3547)
• f70b9b8 sync: documenation for oneshot (#3592)
• 704de8c stream: remove duplicate doc(test(..)) & cfg_attr declarations (#3571)
• 47be928 sync: yield initial value in WatchStream (#3576)
• e06b257 sync: add same_channel method to mpsc Senders (#3532)
• 0867a6f util: add pollable mpsc::Sender (#3490)
• 0d838bf io: fix link to std src in util (#3568)
• Additional commits viewable in compare view

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