Replace usage of deprecated PyO3 downcast #192

Job	Run time
Build wheel for documentation and tests	3m 17s
Run stub generator	34s
Check formatting of pySplashsurf crate	5s
Build wheels for Linux x86_64	3m 24s
Build wheels for macOS aarch64	2m 49s
Build source distribution	22s
Build wheels for Windows i686	7m 20s
Build wheels for macOS x86_64	7m 10s
Build wheels for Windows aarch64	5m 1s
Build wheels for Linux armv7	3m 25s
Build wheels for Linux i686	5m 29s
Build wheels for Windows x86_64	5m 32s
Build wheels for Linux aarch64	3m 51s
Build documentation	26s
Run pytest	2m 41s
Read the Docs trigger	0s
Publish to PyPI	-1s
	51m 25s

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