Migrate to pyproject. #55

Job	Run time
build (3.8, sphinx)	1m 5s
build (3.9, sphinx)	1m 6s
build (3.10, sphinx)	1m 17s
build (3.11, sphinx)	1m 1s
build (3.12, sphinx)	38s
build (3.7, "docutils<0.18" "jinja2<3.1")	1m 7s
build (3.12, sphinx https://github.com/numpy/numpydoc/archive/refs/heads/master.zip, -m "numpydoc")	46s
build (3.12, sphinx https://github.com/brechtm/rinohtype/archive/refs/heads/master.zip, -m "rinoh...	57s
build (3.12, sphinx test/some_cython_module, -m "cython")	13s
build (3.9, "sphinx<4" "jinja2<3.1" "sphinxcontrib-applehelp<1.0.8" "sphinxcontrib-devhelp<1.0.5"...	1m 6s
build (3.10, "sphinx<5" "sphinxcontrib-applehelp<1.0.8" "sphinxcontrib-devhelp<1.0.5" "sphinxcont...	1m 7s
build (3.12, "sphinx<6")	1m 26s
build (3.12, "sphinx<7")	1m 17s
	13m 6s

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