ENH: use `Fraction` for spin values #619

Job	Run time
pytest / Determine job matrix	5s
style / Determine skipped pre-commit hooks	7s
doc / nb / Execute and build documentation	53s
doc / linkcheck / Check external links	43s
style / Run local pre-commit hooks	24s
pytest / Run unit tests (3.10, ubuntu-24.04)	1m 4s
pytest / Run unit tests (3.11, ubuntu-24.04)	1m 11s
pytest / Run unit tests (3.9, ubuntu-24.04)	1m 9s
pytest / Run unit tests (3.12, ubuntu-24.04, qrules)	3m 32s
pytest / Run unit tests (3.9, macos-14)	47s
style / Push changes	4s
doc / Upload to GitHub Pages	0s
	9m 59s

Provide feedback