replace backoff with backon #779

Job	Run time
windows (x86_64-pc-windows-msvc, x64)	15m 41s
sdist	9m 34s
windows (i686-pc-windows-msvc, x86)	14m 10s
macos-aarch64	12m 25s
macos-x86_64	12m 7s
linux (x86_64-unknown-linux-gnu)	7m 54s
linux-arm (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	10m 59s
linux-arm (armv7-unknown-linux-gnueabihf, armv7)	10m 40s
linux (i686-unknown-linux-gnu)	10m 47s
linux-powerpc (powerpc64le-unknown-linux-gnu, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	11m 24s
linux-arm (arm-unknown-linux-musleabihf, arm)	10m 35s
linux-powerpc (powerpc64-unknown-linux-gnu, ppc64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	11m 10s
linux-s390x (s390x-unknown-linux-gnu, s390x)	16m 5s
musllinux (x86_64-unknown-linux-musl)	11m 29s
musllinux (i686-unknown-linux-musl)	9m 49s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	11m 30s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	11m 20s
	3h 17m 39s

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