Prototype support for PEP 739 #17124

Job	Run time
Arch Linux	19m 6s
Arch / PyPy	20m 10s
CUDA (on Arch)	6m 54s
Ubuntu Rolling (gcc, g++)	18m 32s
Fedora	16m 27s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 44s
Gentoo	20m 40s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 56s
OpenSUSE	15m 46s
Ubuntu Rolling (clang, clang++)	25m 2s
Ubuntu Bionic	14m 4s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 16s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	14m 7s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	14m 30s
	3h 48m 14s

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