bump version to 0.21.0 (#201) #181

Job	Run time
Populate cache	16m 8s
Build Python sdist	38s
Build and release (macos-latest, amd64, x86_64, arm64-macos, target/release)	20m 49s
Build and release (windows-latest, amd64, x64, x86_64-windows, .exe, target/release)	27m 39s
Build and release (ubuntu-latest, amd64, x86_64, x86_64-linux, target/release)	15m 8s
Build and release (macos-latest, aarch64, aarch64, arm64-macos, --target aarch64-apple-darwin, aa...	12m 40s
Build and release (ubuntu-latest, aarch64, aarch64, x86_64-linux, --target aarch64-unknown-linux-...	17m 25s
Generate checksums	8s
Update canary release	36s
	1h 51m 11s

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