Bump version to 0.4.20 #539

Job	Run time
macos-aarch64	7m 23s
sdist	8m 25s
macos-x86_64	11m 7s
linux (x86_64-unknown-linux-gnu)	9m 41s
linux-arm (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	9m 43s
windows (x86_64-pc-windows-msvc, x64)	15m 5s
linux (i686-unknown-linux-gnu)	13m 19s
linux-arm (armv7-unknown-linux-gnueabihf, armv7)	9m 20s
windows (i686-pc-windows-msvc, x86)	17m 4s
linux-arm (arm-unknown-linux-musleabihf, arm)	8m 47s
linux-powerpc (powerpc64le-unknown-linux-gnu, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	9m 40s
linux-s390x (s390x-unknown-linux-gnu, s390x)	13m 48s
linux-powerpc (powerpc64-unknown-linux-gnu, ppc64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	8m 43s
musllinux (x86_64-unknown-linux-musl)	9m 9s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	10m 8s
musllinux (i686-unknown-linux-musl)	8m 42s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	9m 35s
musllinux-cross (powerpc64le-unknown-linux-musl, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16, nightly)	7m 14s
	3h 6m 53s

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