CI: Explicit Python version #627

Job	Run time
Check and build entire workspace	22s
Build splashsurf_lib with all features	1m 59s
Run cargo fmt	8s
Build splashsurf_lib with no default features	9s
Build & test release mode (${{ matrix.runner }})	0s
Publish to crates.io	0s
	2m 38s

Provide feedback