Bump version to v0.4.21 #540

Job	Run time
macos-aarch64	1s
macos-x86_64	7m 2s
sdist	7m 29s
linux (x86_64-unknown-linux-gnu)	6m 53s
linux-arm (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	6m 48s
windows (x86_64-pc-windows-msvc, x64)	6m 47s
linux (i686-unknown-linux-gnu)	6m 59s
linux-arm (armv7-unknown-linux-gnueabihf, armv7)	6m 41s
windows (i686-pc-windows-msvc, x86)	6m 47s
linux-arm (arm-unknown-linux-musleabihf, arm)	6m 44s
linux-powerpc (powerpc64le-unknown-linux-gnu, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	6m 36s
linux-s390x (s390x-unknown-linux-gnu, s390x)	6m 25s
musllinux (x86_64-unknown-linux-musl)	6m 26s
linux-powerpc (powerpc64-unknown-linux-gnu, ppc64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	6m 33s
musllinux (i686-unknown-linux-musl)	6m 28s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	6m 24s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	6m 23s
musllinux-cross (powerpc64le-unknown-linux-musl, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16, nightly)	6m 23s
	1h 53m 49s

Provide feedback