Refactor `pint.fitter` #4830

Job	Run time
ubuntu-latest, ephemeris_connection	59s
macos-latest-py313	1h 32m 2s
ubuntu-latest, black	17s
ubuntu-latest, py313-test-cov	1h 0m 51s
ubuntu-latest, notebooks	4m 53s
ubuntu-latest, oldestdeps	28m 31s
	3h 7m 33s

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