ENH: use `Fraction` for spin values #625

Job	Run time
pytest / Determine job matrix	6s
style / Determine skipped pre-commit hooks	9s
doc / linkcheck / Check external links	44s
doc / nb / Execute and build documentation	50s
pytest / Run unit tests (3.10, ubuntu-24.04)	1m 14s
pytest / Run unit tests (3.11, ubuntu-24.04)	1m 4s
pytest / Run unit tests (3.9, ubuntu-24.04)	1m 4s
pytest / Run unit tests (3.12, ubuntu-24.04, qrules)	3m 32s
pytest / Run unit tests (3.9, macos-14)	1m 2s
style / Run local pre-commit hooks	47s
doc / Upload to GitHub Pages	0s
style / Push changes	0s
	10m 32s

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