Update tokio requirement from 0.1.12 to 0.2.9

[dependabot-preview[bot]]

Updates the requirements on tokio to permit the latest version.

Release notes

Sourced from tokio's releases.

Tokio v0.2.9

Includes only fixes.

Fixes

• AsyncSeek impl for File (#1986).
• rt: shutdown deadlock in threaded_scheduler (#2074, #2082).
• rt: memory ordering when dropping JoinHandle (#2044).
• docs: misc API documentation fixes and improvements.

Commits

• c7c74a5 chore: prepare v0.2.9 release (#2084)
• b34a849 docs: fix runtime creation doc in tokio::runtime (#2073)
• a7a79f2 rt: use release ordering in drop_join_handle_fast (#2044)
• bd28a7a docs: fix typo and issue reference (#2080)
• 275769b rt: fix shutdown deadlock in threaded scheduler (#2082)
• b70615b docs: document feature flags (#2081)
• 6406328 rt: fix threaded scheduler shutdown deadlock (#2074)
• f28c9f0 Fix Seek adapter and AsyncSeek error handling for File
• 7ee5542 doc: fix old notes regarding examples and async/await (#2071)
• 7fb5431 macros: fix breaking changes (#2069)
• Additional commits viewable in compare view

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[dependabot-preview[bot]]

Superseded by #93.