Remove hot uses of pathlib #17824

Job	Run time
Arch / PyPy	22m 14s
Ubuntu Rolling (gcc, g++)	22m 0s
Arch Linux	21m 38s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	15m 29s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	16m 12s
CUDA (on Arch)	7m 11s
Ubuntu Rolling (clang, clang++)	26m 11s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	18m 25s
Fedora	20m 14s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	14m 12s
Gentoo	24m 49s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 36s
OpenSUSE	18m 48s
Ubuntu Bionic	15m 59s
	4h 16m 58s

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