Replace usage of deprecated PyO3 downcast #610

Job	Run time
Run cargo fmt	8s
Build splashsurf_lib with all features	29s
Build splashsurf_lib with no default features	20s
Check and build entire workspace	1m 26s
Build & test release mode (ubuntu-latest)	3m 8s
Build & test release mode (macos-13)	15m 3s
Build & test release mode (macos-14)	3m 39s
Build & test release mode (windows-latest)	8m 4s
Build & test release mode (ubuntu-24.04-arm)	2m 46s
Publish to crates.io	0s
	35m 3s

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