improve smart flattening #380

Job	Run time
unavailable	1m 59s
build (ubuntu-latest, linux, x86-64)	2m 53s
build (macos-latest, darwin, aarch64)	1m 46s
id	4m 1s
test (darwin+aarch64)	5m 40s
test	3m 7s
audit (macos-latest)	1m 59s
audit (ubuntu-latest)	2m 5s
bottle (gz, darwin+aarch64)	1m 54s
bottle (gz, linux+x86-64)	1m 36s
bottle (xz, linux+x86-64)	1m 37s
bottle (xz, darwin+aarch64)	1m 41s
	30m 18s

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