ENH: use `Fraction` for spin values #621

Job	Run time
pytest / Determine job matrix	10s
style / Determine skipped pre-commit hooks	5s
doc / nb / Execute and build documentation	49s
doc / linkcheck / Check external links	49s
style / Run local pre-commit hooks	46s
pytest / Run unit tests (3.10, ubuntu-24.04)	1m 9s
pytest / Run unit tests (3.11, ubuntu-24.04)	1m 15s
pytest / Run unit tests (3.9, ubuntu-24.04)	1m 11s
pytest / Run unit tests (3.12, ubuntu-24.04, qrules)	3m 27s
pytest / Run unit tests (3.9, macos-14)	57s
doc / Upload to GitHub Pages	0s
style / Push changes	0s
	10m 38s

Provide feedback