Update tokio requirement from 0.2 to 1.2

[dependabot-preview[bot]]

Updates the requirements on tokio to permit the latest version.

Release notes

Sourced from tokio's releases.

Tokio v1.2.0

Added

• signal: make Signal::poll_recv method public (#3383)

Fixed

• time: make test-util paused time fully deterministic (#3492)

Documented

• sync: link to new broadcast and watch wrappers (#3504)

Commits

• 572a897 chore: prepare tokio v1.2.0 (#3506)
• d41882e chore: prepare tokio-macros 1.1.0 (#3505)
• 23fdc2b chore: prepare tokio-stream 0.1.3 (#3507)
• 0a04954 stream: update features and doc for broadcast/watch stream (#3504)
• fcb6d04 time: make test-util paused time fully deterministic (#3492)
• 1c1e0e3 tokio-stream: add wrapper for broadcast and watch (#3384)
• 77ca8a9 signal: make Signal::poll_recv method public (#3383)
• 3e5a0a7 runtime: fix a spelling mistake (#3503)
• cc97fb8 Suppress deprecated warnings for spin_loop_hint (#3497)
• 60d8884 chore: prepare tokio-util v0.6.3 (#3488)
• Additional commits viewable in compare view

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[dependabot-preview[bot]]

Superseded by #27.