Remove non-windows requirement for `async-io`/`tokio` #266

Job	Run time
Build Channels Linux (stable)	28s
Build Channels Linux (1.70.0)	29s
Build Channels OpenBSD (stable)	3m 7s
Build Channels macOS (stable, macos-14)	22s
Build Channels DragonFlyBSD (stable)	3m 20s
Build Channels Windows (stable, x86_64-pc-windows-msvc)	1m 40s
Style check	21s
Build Channels Windows (1.70.0, x86_64-pc-windows-msvc)	54s
Build Channels FreeBSD (stable)	3m 46s
Build Channels NetBSD (stable)	2m 39s
Build Channels macOS (1.70.0, macos-14)	24s
Build Channels FreeBSD (1.70.0)	3m 11s
Build Channels NetBSD (1.70.0)	3m 22s
Check #[cfg]s	14s
success	1s
	24m 18s

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