Support for pickling sentinel objects as singletons #2711

Job	Run time
Run tests (3.10.4)	21s
Run tests (3.9.12)	20s
Run tests (3.14)	15s
Run tests (3.11.0)	20s
Run tests (3.12.0)	26s
Run tests (3.11)	9s
Run tests (3.9)	19s
Run tests (3.12)	14s
Run tests (3.13)	18s
Run tests (3.13.0)	19s
Run tests (3.10)	14s
Run tests (pypy3.11)	22s
Run tests (pypy3.10)	20s
Run tests (pypy3.9)	21s
Create an issue if daily tests failed	0s
	4m 18s

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