Refactor `pint.fitter` #4815

Job	Run time
macos-latest-py313	1h 32m 38s
ubuntu-latest, ephemeris_connection	1m 11s
ubuntu-latest, black	20s
ubuntu-latest, py313-test-cov	1h 29m 6s
ubuntu-latest, notebooks	3m 19s
ubuntu-latest, oldestdeps	28m 29s
	3h 35m 3s

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