Fix df.incore #938

Job	Run time
macos-build (3.7)	10m 23s
linux-build (ubuntu-20.04, 3.7)	1h 1m 43s
linux-build-aarch64 (ubuntu-20.04, cp37-cp37m)	11m 7s
linux-build (ubuntu-20.04, 3.8)	1h 3m 23s
linux-build-aarch64 (ubuntu-20.04, cp38-cp38)	11m 25s
linux-build (ubuntu-20.04, 3.9)	1h 2m 33s
linux-build-aarch64 (ubuntu-20.04, cp39-cp39)	10m 59s
linux-build (ubuntu-20.04, 3.10)	1h 3m 8s
linux-build (ubuntu-20.04, 3.11)	1h 14m 30s
	6h 9m 11s

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