Update tokio requirement from 0.1.7 to 0.2.21

[dependabot-preview[bot]]

Updates the requirements on tokio to permit the latest version.

Release notes

Sourced from tokio's releases.

Tokio v0.2.21

Bug fixes and API polish.

Fixes

• macros: disambiguate built-in #[test] attribute in macro expansion (#2503)
• rt: LocalSet and task budgeting (#2462).
• rt: task budgeting with block_in_place (#2502).
• sync: release broadcast channel memory without sending a value (#2509).
• time: notify when resetting a Delay to a time in the past (#2290).

Added

• io: get_mut, get_ref, and into_inner to Lines (#2450).
• io: mio::Ready argument to PollEvented (#2419).
• os: illumos support (#2486).
• rt: Handle::spawn_blocking (#2501).
• sync: OwnedMutexGuard for Arc<Mutex<T>> (#2455).

Commits

• 02661ba chore: prepare v0.2.21 release (#2530)
• fb7dfcf sync: use intrusive list strategy for broadcast (#2509)
• a32f918 chore: change norun to no_run (#2518)
• 221f421 codec: rewrite of codec::Framed (#2368)
• 1cc0168 macros: disambiguate the built-in #[test] attribute in macro expansion (#2503)
• 67220ea tokio: add support for illumos target (#2486)
• 3ba818a io: add doc warning about concurrently calling poll_read/write_ready (#2439)
• 6aeeeff io: add mio::Ready argument to PollEvented (#2419)
• a75fe38 stream: fix documentation on filter_map (#2511)
• adce911 doc: add link fragments to CONTRIBUTING.md (#2507)
• Additional commits viewable in compare view

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[dependabot-preview[bot]]

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