Update Cython to v3.1 #1208

Job	Run time
Run tests with coverage	2m 50s
Run tests with different pydantic versions	3m 8s
Run tests (3.10)	1m 55s
Run tests (3.12)	1m 51s
Run tests (3.9)	1m 46s
Run tests (3.8)	1m 53s
Run tests (3.13)	1m 48s
Run linters (mypy)	1m 35s
Run linters (flake8)	1m 27s
Run tests (3.11)	1m 43s
Run linters (pylint)	1m 33s
Run linters (pydocstyle)	1m 30s
	22m 59s

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