Support for pickling sentinel objects as singletons #2722

Job	Run time
Run tests (3.9.12)	20s
Run tests (3.10)	21s
Run tests (3.11)	12s
Run tests (3.11.0)	20s
Run tests (3.13.0)	21s
Run tests (3.12.0)	25s
Run tests (3.13)	16s
Run tests (3.10.4)	18s
Run tests (pypy3.10)	23s
Run tests (3.12)	18s
Run tests (3.9)	19s
Run tests (pypy3.9)	24s
Run tests (3.14)	16s
Run tests (pypy3.11)	21s
Create an issue if daily tests failed	0s
	4m 34s

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