chore(deps): update tokio requirement from 0.2.6 to 0.3.1

[dependabot-preview[bot]]

Updates the requirements on tokio to permit the latest version.

Release notes

Sourced from tokio's releases.

Tokio v0.3.1

This release fixes a use-after-free in the IO driver. Additionally, the read_buf and write_buf methods have been added back to the IO traits, as the bytes crate is now on track to reach version 1.0 together with Tokio.

Fixed

• net: fix use-after-free (#3019).
• fs: ensure buffered data is written on shutdown (#3009).

Added

• io: copy_buf() (#2884).
• io: AsyncReadExt::read_buf(), AsyncReadExt::write_buf() for working with Buf/BufMut (#3003).
• rt: Runtime::spawn_blocking() (#2980).
• sync: watch::Sender::is_closed() (#2991).

Commits

• d14cbf9 chore: prepare v0.3.1 release (#3021)
• 8bfb1c9 sync: revert Clone impl for broadcast::Receiver (#3020)
• b48fec9 net: fix use-after-free in slab compaction (#3019)
• 8dbc3c7 io: add AsyncReadExt::read_buf (#3003)
• 7fbfa9b tokio: deduplicate spawn_blocking (#3017)
• 7d7b79e sync: add is_closed method to watch sender (#2991)
• 8f37544 io: explain how to determine number of bytes read in AsyncRead (#3011)
• 43d0714 sync: remove extra clone in Semaphore::[try_]acquire_owned (#3015)
• 16e272e fs: flush on shutdown (#3009)
• 6d99e1c util: prevent read buffer from being swapped during a read_poll (#2993)
• Additional commits viewable in compare view

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[dependabot-preview[bot]]

Superseded by #74.