ENH: use `Fraction` for spin values #618

Job	Run time
style / Determine skipped pre-commit hooks	9s
pytest / Determine job matrix	5s
doc / nb / Execute and build documentation	1m 29s
doc / linkcheck / Check external links	44s
pytest / Run unit tests (3.10, ubuntu-24.04)	1m 3s
pytest / Run unit tests (3.11, ubuntu-24.04)	1m 9s
pytest / Run unit tests (3.9, ubuntu-24.04)	1m 12s
pytest / Run unit tests (3.12, ubuntu-24.04, qrules)	3m 22s
pytest / Run unit tests (3.9, macos-14)	56s
style / Run local pre-commit hooks	24s
style / Push changes	7s
doc / Upload to GitHub Pages	0s
	10m 40s

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