Avoid `RemovedInSphinx80Warning` in path-manipulation code. #912

Job	Run time
build (3.7, 4.0.3)	17s
build (3.7, 4.1.2)	22s
build (3.7, 4.2.0)	26s
build (3.7, 4.3.2)	17s
build (3.7, 4.5.0)	27s
build (3.7, 5.0.2)	21s
build (3.7, 5.1.1)	24s
build (3.7, 5.3.0)	15s
build (3.8, 4.0.3)	28s
build (3.8, 4.1.2)	18s
build (3.8, 4.2.0)	27s
build (3.8, 4.3.2)	28s
build (3.8, 4.5.0)	26s
build (3.8, 5.0.2)	21s
build (3.8, 5.1.1)	17s
build (3.8, 5.3.0)	15s
build (3.8, 6.1.3)	22s
build (3.8, git+https://github.com/sphinx-doc/sphinx.git@master)	16s
build (3.9, 4.0.3)	27s
build (3.9, 4.1.2)	20s
build (3.9, 4.2.0)	21s
build (3.9, 4.3.2)	30s
build (3.9, 4.5.0)	29s
build (3.9, 5.0.2)	21s
build (3.9, 5.1.1)	18s
build (3.9, 5.3.0)	38s
build (3.9, 6.1.3)	15s
build (3.9, git+https://github.com/sphinx-doc/sphinx.git@master)	7s
build (3.10, 4.2.0)	22s
build (3.10, 4.3.2)	35s
build (3.10, 4.5.0)	25s
build (3.10, 5.0.2)	28s
build (3.10, 5.1.1)	22s
build (3.10, 5.3.0)	19s
build (3.10, 6.1.3)	20s
build (3.10, git+https://github.com/sphinx-doc/sphinx.git@master)	23s
	13m 27s

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