Refactor `pint.fitter` #4826

Job	Run time
macos-latest-py313	1h 12m 17s
ubuntu-latest, ephemeris_connection	1m 7s
ubuntu-latest, black	21s
ubuntu-latest, py313-test-cov	1h 2m 23s
ubuntu-latest, notebooks	5m 5s
ubuntu-latest, oldestdeps	29m 44s
	2h 50m 57s

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