ENH: use `Fraction` for spin values #623

Job	Run time
style / Determine skipped pre-commit hooks	8s
pytest / Determine job matrix	4s
doc / linkcheck / Check external links	59s
doc / nb / Execute and build documentation	55s
pytest / Run unit tests (3.10, ubuntu-24.04)	1m 10s
pytest / Run unit tests (3.11, ubuntu-24.04)	1m 7s
pytest / Run unit tests (3.9, ubuntu-24.04)	1m 6s
pytest / Run unit tests (3.12, ubuntu-24.04, qrules)	3m 38s
pytest / Run unit tests (3.9, macos-14)	49s
style / Run local pre-commit hooks	45s
doc / Upload to GitHub Pages	0s
style / Push changes	0s
	10m 41s

Provide feedback