Refactor `pint.fitter` #4863

Job	Run time
macos-latest-py313	1h 9m 16s
ubuntu-latest, ephemeris_connection	1m 33s
ubuntu-latest, black	18s
ubuntu-latest, py313-test-cov	1h 0m 6s
ubuntu-latest, notebooks	5m 23s
ubuntu-latest, oldestdeps	29m 54s
	2h 46m 30s

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