Support for pickling sentinel objects as singletons #2710

Job	Run time
Run tests (3.11)	8s
Run tests (3.11.0)	24s
Run tests (3.10)	22s
Run tests (3.10.4)	19s
Run tests (3.9.12)	19s
Run tests (3.9)	18s
Run tests (3.12.0)	25s
Run tests (pypy3.11)	23s
Run tests (pypy3.10)	23s
Run tests (pypy3.9)	20s
Run tests (3.12)	16s
Run tests (3.13)	16s
Run tests (3.13.0)	22s
Run tests (3.14)	18s
Create an issue if daily tests failed	-1s
	4m 32s

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