ENH: use `Fraction` for spin values #620

Job	Run time
style / Determine skipped pre-commit hooks	6s
pytest / Determine job matrix	15s
doc / nb / Execute and build documentation	53s
doc / linkcheck / Check external links	48s
style / Run local pre-commit hooks	24s
pytest / Run unit tests (3.10, ubuntu-24.04)	1m 13s
pytest / Run unit tests (3.11, ubuntu-24.04)	1m 14s
pytest / Run unit tests (3.9, ubuntu-24.04)	1m 11s
pytest / Run unit tests (3.12, ubuntu-24.04, qrules)	3m 36s
pytest / Run unit tests (3.9, macos-14)	49s
style / Push changes	4s
doc / Upload to GitHub Pages	0s
	10m 33s

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