Bump version to 0.4.20 #538

Job	Run time
macos-x86_64	8m 19s
windows (x86_64-pc-windows-msvc, x64)	8m 17s
windows (i686-pc-windows-msvc, x86)	8m 18s
sdist	8m 15s
macos-aarch64	7m 18s
linux (x86_64-unknown-linux-gnu)	8m 21s
linux-arm (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	8m 20s
linux-powerpc (powerpc64le-unknown-linux-gnu, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	8m 20s
linux (i686-unknown-linux-gnu)	8m 19s
linux-arm (armv7-unknown-linux-gnueabihf, armv7)	8m 18s
linux-powerpc (powerpc64-unknown-linux-gnu, ppc64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	8m 21s
linux-arm (arm-unknown-linux-musleabihf, arm)	8m 18s
linux-s390x (s390x-unknown-linux-gnu, s390x)	8m 19s
musllinux (x86_64-unknown-linux-musl)	8m 18s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	8m 19s
musllinux (i686-unknown-linux-musl)	8m 20s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	8m 19s
musllinux-cross (powerpc64le-unknown-linux-musl, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16, nightly)	7m 33s
	2h 27m 52s

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