Avoid `RemovedInSphinx80Warning` in path-manipulation code. (#977) #913

Job	Run time
build (3.7, 4.0.3)	17s
build (3.7, 4.1.2)	23s
build (3.7, 4.2.0)	24s
build (3.7, 4.3.2)	25s
build (3.7, 4.5.0)	22s
build (3.7, 5.0.2)	20s
build (3.7, 5.1.1)	21s
build (3.7, 5.3.0)	26s
build (3.8, 4.0.3)	17s
build (3.8, 4.1.2)	20s
build (3.8, 4.2.0)	23s
build (3.8, 4.3.2)	25s
build (3.8, 4.5.0)	25s
build (3.8, 5.0.2)	21s
build (3.8, 5.1.1)	22s
build (3.8, 5.3.0)	21s
build (3.8, 6.1.3)	33s
build (3.8, git+https://github.com/sphinx-doc/sphinx.git@master)	14s
build (3.9, 4.0.3)	24s
build (3.9, 4.1.2)	22s
build (3.9, 4.2.0)	21s
build (3.9, 4.3.2)	26s
build (3.9, 4.5.0)	18s
build (3.9, 5.0.2)	21s
build (3.9, 5.1.1)	24s
build (3.9, 5.3.0)	18s
build (3.9, 6.1.3)	16s
build (3.9, git+https://github.com/sphinx-doc/sphinx.git@master)	19s
build (3.10, 4.2.0)	24s
build (3.10, 4.3.2)	24s
build (3.10, 4.5.0)	18s
build (3.10, 5.0.2)	24s
build (3.10, 5.1.1)	23s
build (3.10, 5.3.0)	17s
build (3.10, 6.1.3)	16s
build (3.10, git+https://github.com/sphinx-doc/sphinx.git@master)	23s
	12m 57s

Provide feedback