ENH: use `Fraction` for spin values #622

Job	Run time
pytest / Determine job matrix	7s
style / Determine skipped pre-commit hooks	7s
doc / linkcheck / Check external links	39s
doc / nb / Execute and build documentation	53s
pytest / Run unit tests (3.10, ubuntu-24.04)	1m 20s
pytest / Run unit tests (3.11, ubuntu-24.04)	1m 13s
pytest / Run unit tests (3.9, ubuntu-24.04)	1m 12s
pytest / Run unit tests (3.12, ubuntu-24.04, qrules)	3m 28s
pytest / Run unit tests (3.9, macos-14)	53s
style / Run local pre-commit hooks	51s
doc / Upload to GitHub Pages	0s
style / Push changes	0s
	10m 43s

Provide feedback