drop 3.9, move to pyproject etc #401

Job	Run time
core / Load tox environments	4s
core / py312 (ubuntu-latest)	2m 7s
test / Load tox environments	7s
test / py310 (macos-latest)	2m 21s
test / py311 (windows-latest)	2m 44s
test / py310-oldestdeps (ubuntu-latest)	1m 51s
publish	0s
online / Load tox environments	7s
docs / Load tox environments	5s
docs / build_docs (ubuntu-latest)	3m 16s
online / py312-online (ubuntu-latest)	4m 1s
conda	3m 36s
devdeps / Load tox environments	10s
devdeps / py312-devdeps (ubuntu-latest)	4m 47s
	25m 16s

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