Bump version to 0.6.8 #110

Job	Run time
macos-x86_64	5m 46s
macos-aarch64	3m 41s
linux (x86_64-unknown-linux-gnu)	5m 3s
linux-cross (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	5m 47s
linux (i686-unknown-linux-gnu)	7m 25s
musllinux (x86_64-unknown-linux-musl)	4m 57s
linux-cross (armv7-unknown-linux-gnueabihf, armv7)	5m 25s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	5m 29s
musllinux (i686-unknown-linux-musl)	4m 28s
linux-cross (s390x-unknown-linux-gnu, s390x)	5m 40s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	5m 11s
sdist	4m 28s
linux-cross (powerpc64le-unknown-linux-gnu, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	5m 30s
linux-cross (powerpc64-unknown-linux-gnu, ppc64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	5m 22s
windows (x86_64-pc-windows-msvc, x64)	7m 52s
linux-cross (arm-unknown-linux-musleabihf, arm)	4m 33s
windows (i686-pc-windows-msvc, x86)	8m 31s
windows (aarch64-pc-windows-msvc, x64)	9m 26s
	1h 44m 34s

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