[SkyMarshal] Drop `six` dependency #394

Job	Run time
test-on-ubuntu (ubuntu-20.04, python3.8, gcc-11, g++-11, g++-11, "ppa:ubuntu-toolchain-r/test")	25m 32s
test-on-ubuntu (ubuntu-20.04, python3.9, gcc-11, g++-11, g++-11, "ppa:ubuntu-toolchain-r/test")	26m 7s
test-on-ubuntu (ubuntu-20.04, python3.10, gcc-11, g++-11, g++-11, "ppa:ubuntu-toolchain-r/test")	24m 38s
test-on-ubuntu (ubuntu-20.04, python3.11, gcc-11, g++-11, g++-11, "ppa:ubuntu-toolchain-r/test")	24m 40s
test-on-ubuntu (ubuntu-20.04, python3.12, gcc-11, g++-11, g++-11, "ppa:ubuntu-toolchain-r/test")	24m 55s
test-on-ubuntu (ubuntu-22.04, python3.8, gcc-11, g++-11, g++-11, "ppa:ubuntu-toolchain-r/test")	25m 56s
test-on-ubuntu (ubuntu-22.04, python3.9, gcc-11, g++-11, g++-11, "ppa:ubuntu-toolchain-r/test")	25m 53s
test-on-ubuntu (ubuntu-22.04, python3.10, gcc-11, g++-11, g++-11, "ppa:ubuntu-toolchain-r/test")	25m 40s
test-on-ubuntu (ubuntu-22.04, python3.11, gcc-11, g++-11, g++-11, "ppa:ubuntu-toolchain-r/test")	26m 29s
test-on-ubuntu (ubuntu-22.04, python3.12, gcc-11, g++-11, g++-11, "ppa:ubuntu-toolchain-r/test")	26m 36s
test-on-ubuntu (ubuntu-20.04, python3.8, gcc-5, g++-5, g++-5, "deb http://us.archive.ubuntu.com/u...	25m 25s
test-on-ubuntu (ubuntu-20.04, python3.8, clang-7, clang++-7, clang-7)	25m 54s
test-on-ubuntu (ubuntu-22.04, python3.10, clang-15, clang++-15, clang-15)	28m 36s
	5h 36m 21s

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