Fix `sphinx` installation in GitHub Actions (#953) #920

Job	Run time
build (3.7, 4.0.3)	8s
build (3.7, 4.1.2)	13s
build (3.7, 4.2.0)	6s
build (3.7, 4.3.2)	8s
build (3.7, 4.5.0)	13s
build (3.7, 5.0.2)	7s
build (3.7, 5.1.1)	9s
build (3.7, 5.3.0)	11s
build (3.8, 4.0.3)	24s
build (3.8, 4.1.2)	25s
build (3.8, 4.2.0)	17s
build (3.8, 4.3.2)	24s
build (3.8, 4.5.0)	18s
build (3.8, 5.0.2)	17s
build (3.8, 5.1.1)	20s
build (3.8, 5.3.0)	25s
build (3.8, 6.1.3)	20s
build (3.8, git+https://github.com/sphinx-doc/sphinx.git@master)	18s
build (3.9, 4.0.3)	17s
build (3.9, 4.1.2)	20s
build (3.9, 4.2.0)	16s
build (3.9, 4.3.2)	26s
build (3.9, 4.5.0)	18s
build (3.9, 5.0.2)	19s
build (3.9, 5.1.1)	15s
build (3.9, 5.3.0)	19s
build (3.9, 6.1.3)	17s
build (3.9, git+https://github.com/sphinx-doc/sphinx.git@master)	14s
build (3.10, 4.2.0)	17s
build (3.10, 4.3.2)	20s
build (3.10, 4.5.0)	21s
build (3.10, 5.0.2)	19s
build (3.10, 5.1.1)	21s
build (3.10, 5.3.0)	16s
build (3.10, 6.1.3)	16s
build (3.10, git+https://github.com/sphinx-doc/sphinx.git@master)	17s
	10m 11s

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