chore: release v0.4.21 #392

Job	Run time
cargo test --shallow-minimal-versions	1m 3s
cargo test minimal-versions	55s
cargo test (stable, i686-unknown-linux-gnu, ubuntu-latest)	57s
cargo test (stable, i686-pc-windows-msvc, windows-latest)	2m 58s
cargo test (stable, x86_64-unknown-linux-gnu, ubuntu-latest)	41s
cargo test (stable, x86_64-pc-windows-msvc, windows-latest)	2m 59s
cargo hack check --all-targets --feature-powerset	1m 2s
cargo test (stable, x86_64-apple-darwin, macos-latest)	54s
cargo hack check --feature-powerset	33s
	12m 2s

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