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alter _flatten_list using flat path #440

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Job	Run time
(basic dependencies) Python 3.9	24s
(basic dependencies) Python 3.10	28s
(basic dependencies) Python 3.11	24s
(basic dependencies) Python 3.12	22s
(Pandas only) Python 3.9	29s
(Pandas only) Python 3.10	28s
(Pandas only) Python 3.11	25s
(Pandas only) Python 3.12	25s
(Pandas & GeoPandas) Python 3.9	30s
(Pandas & GeoPandas) Python 3.10	31s
(Pandas & GeoPandas) Python 3.11	33s
(Pandas & GeoPandas) Python 3.12	30s
	5m 29s