Add Python 3.12 to test matrix. #1305

Job	Run time
Check lint and rustfmt	2m 26s
Check fuzz targets	22s
Check rustdoc WASM target	17s
Run tests	4m 21s
Run JS linters	2m 3s
Python tests and maturin build (3.9)	1m 19s
pre-publish checks	0s
WASM tests	1m 4s
Python tests and maturin build (3.10)	1m 8s
Python tests and maturin build (3.11)	1m 11s
Python tests and maturin build (3.12)	2m 9s
All CI stages	1s
pre-publish checks	0s
publish trustfall_filetests_macros	0s
publish trustfall_core	0s
publish python trustfall	0s
publish trustfall_derive	0s
publish trustfall	0s
publish trustfall_stubgen	0s
	16m 21s

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