Add testing on Python 3.12 #506

Job	Run time
tests / Load tox environments	11s
tests / py38-test-oldestdeps (macos-latest)	13m 41s
tests / py39-test (macos-latest)	16m 57s
tests / py310-test (macos-latest)	16m 30s
tests / py311-test (macos-latest)	19m 26s
tests / py312-test (macos-latest)	4m 1s
tests / py38-test-oldestdeps (ubuntu-latest)	12m 44s
tests / py38-test (ubuntu-latest)	11m 33s
tests / py39-test (ubuntu-latest)	13m 32s
tests / py310-test (ubuntu-latest)	10m 42s
tests / py311-test (ubuntu-latest)	12m 52s
tests / py312-test (ubuntu-latest)	2m 2s
tests / py312-test-devdeps (ubuntu-latest)	2m 59s
tests / py38-test-oldestdeps (windows-latest)	12m 42s
tests / py39-test (windows-latest)	14m 14s
tests / py310-test (windows-latest)	13m 28s
tests / py311-test (windows-latest)	14m 2s
tests / py312-test (windows-latest)	3m 58s
publish	0s
	3h 15m 34s

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