ci: use 3.12 #634

Job	Run time
coverage	5m 48s
docs	2m 58s
lint	2m 2s
Rust unit tests (ubuntu-latest)	3m 21s
Rustfmt	9s
Rust unit tests (macos-latest)	3m 45s
typecheck	2m 53s
Rust unit tests (windows-latest)	4m 51s
test (macos-latest, 3.8)	6m 58s
test (macos-latest, 3.9)	7m 1s
test (macos-latest, 3.10)	5m 39s
test (macos-latest, 3.11)	7m 13s
test (macos-latest, 3.12)	10m 18s
test (ubuntu-latest, 3.8)	4m 30s
test (ubuntu-latest, 3.9)	4m 27s
test (ubuntu-latest, 3.10)	5m 12s
test (ubuntu-latest, 3.11)	4m 55s
test (ubuntu-latest, 3.12)	6m 19s
test (windows-latest, 3.8)	7m 39s
test (windows-latest, 3.9)	7m 24s
test (windows-latest, 3.10)	7m 33s
test (windows-latest, 3.11)	11m 31s
test (windows-latest, 3.12)	11m 51s
	2h 14m 17s

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