Support for pickling sentinel objects as singletons #2717

Job	Run time
Run tests (3.10)	14s
Run tests (3.10.4)	25s
Run tests (3.11)	10s
Run tests (3.9)	22s
Run tests (3.11.0)	23s
Run tests (3.12.0)	20s
Run tests (3.9.12)	20s
Run tests (3.13.0)	20s
Run tests (3.14)	17s
Run tests (pypy3.10)	22s
Run tests (3.13)	17s
Run tests (3.12)	18s
Run tests (pypy3.11)	18s
Run tests (pypy3.9)	20s
Create an issue if daily tests failed	0s
	4m 26s

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