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 v1.0.3

Fixed

• io: memory leak during shutdown (#3477).

Commits

• 5d0a81f io: fix memory leak during shutdown (#3477)
• 5d35c90 chore: release tokio v1.0.2 (#3429)
• ad8fb41 io: fix unsoundness in read_to_end (#3428)
• 2330edc chore: prepare tokio 1.0.1 release (#3347)
• fe2664a sync: remove RwLockWriteGuard::map and RwLockWriteGuard::try_map (#3345)
• 1eefbc2 docs: remove stream feature from docs (#3335)
• 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)
• Additional commits viewable in compare view

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