Support for pickling sentinel objects as singletons #2727

Job	Run time
Run tests (3.10)	18s
Run tests (3.11)	16s
Run tests (3.12)	17s
Run tests (3.12.0)	26s
Run tests (3.14)	18s
Run tests (3.11.0)	20s
Run tests (pypy3.9)	22s
Run tests (3.13.0)	17s
Run tests (3.9)	23s
Run tests (3.10.4)	24s
Run tests (3.9.12)	19s
Run tests (3.13)	16s
Run tests (pypy3.10)	22s
Run tests (pypy3.11)	25s
Create an issue if daily tests failed	0s
	4m 43s

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