Use PEP 661 `Sentinel` for internal sentinel #2323

Job	Run time
Run tests (3.9.12)	18s
Run tests (3.9)	9s
Run tests (3.10)	10s
Run tests (3.10.4)	18s
Run tests (3.12.0)	16s
Run tests (3.11)	13s
Run tests (3.11.0)	20s
Run tests (3.12)	11s
Run tests (3.13)	11s
Run tests (pypy3.10)	9s
Run tests (3.13.0)	19s
Run tests (pypy3.11)	10s
Run tests (pypy3.9)	10s
Run tests (3.14)	18s
Report coverage	19s
Create an issue if daily tests failed	0s
	3m 31s

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