Bump version to 0.6.8 #109

Job	Run time
macos-aarch64	43s
macos-x86_64	44s
sdist	56s
linux (x86_64-unknown-linux-gnu)	45s
musllinux (x86_64-unknown-linux-musl)	44s
linux (i686-unknown-linux-gnu)	45s
linux-cross (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	45s
musllinux (i686-unknown-linux-musl)	44s
linux-cross (armv7-unknown-linux-gnueabihf, armv7)	43s
linux-cross (s390x-unknown-linux-gnu, s390x)	40s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	43s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	42s
linux-cross (powerpc64le-unknown-linux-gnu, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	43s
windows (x86_64-pc-windows-msvc, x64)	39s
linux-cross (powerpc64-unknown-linux-gnu, ppc64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	42s
windows (i686-pc-windows-msvc, x86)	39s
linux-cross (arm-unknown-linux-musleabihf, arm)	41s
windows (aarch64-pc-windows-msvc, x64)	39s
	12m 57s

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