Fix _vhf.direct_mapdm #945

Job	Run time
macos-build (3.7)	1h 19m 18s
linux-build-aarch64 (ubuntu-20.04, cp37-cp37m)	9m 36s
linux-build (ubuntu-20.04, 3.7)	1h 1m 38s
linux-build-aarch64 (ubuntu-20.04, cp38-cp38)	10m 49s
linux-build (ubuntu-20.04, 3.8)	1h 2m 35s
linux-build-aarch64 (ubuntu-20.04, cp39-cp39)	12m 38s
linux-build (ubuntu-20.04, 3.9)	1h 3m 21s
linux-build (ubuntu-20.04, 3.10)	1h 15m 21s
linux-build (ubuntu-20.04, 3.11)	1h 5m 5s
	7h 20m 21s

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