Use `sha2` to validate Python release checksums #400

Job	Run time
Build with maturin	1m 53s
Clippy lints	30s
Formatting check	21s
Build and test (ubuntu-latest)	2m 47s
Build and test (windows-latest)	5m 47s
Build and test (macos-latest)	4m 27s
	15m 45s

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