Bump version to v0.3.1 #442

Job	Run time
linux-powerpc (powerpc64le-unknown-linux-gnu, ppc64le)	9m 4s
linux-arm (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	8m 36s
linux-s390x (s390x-unknown-linux-gnu, s390x)	11m 46s
linux-arm (armv7-unknown-linux-gnueabihf, armv7)	8m 33s
linux (x86_64-unknown-linux-gnu)	8m 2s
linux-arm (arm-unknown-linux-musleabihf, arm)	7m 57s
linux-powerpc (powerpc64-unknown-linux-gnu, ppc64)	8m 45s
macos-aarch64	7m 12s
linux (i686-unknown-linux-gnu)	11m 43s
macos-x86_64	21m 0s
sdist	7m 11s
windows (x86_64-pc-windows-msvc, x64)	13m 28s
musllinux (x86_64-unknown-linux-musl)	8m 3s
windows (i686-pc-windows-msvc, x86)	14m 7s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	8m 56s
musllinux (i686-unknown-linux-musl)	7m 37s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	8m 14s
musllinux-cross (powerpc64le-unknown-linux-musl, ppc64le, nightly)	7m 17s
	2h 57m 31s

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