Support for pickling sentinel objects as singletons #2712

Job	Run time
Run tests (3.10)	16s
Run tests (3.12.0)	21s
Run tests (3.13)	15s
Run tests (3.12)	19s
Run tests (3.9)	19s
Run tests (3.9.12)	19s
Run tests (3.11)	8s
Run tests (3.13.0)	20s
Run tests (3.10.4)	19s
Run tests (pypy3.10)	25s
Run tests (pypy3.11)	21s
Run tests (3.14)	24s
Run tests (pypy3.9)	25s
Run tests (3.11.0)	20s
Create an issue if daily tests failed	0s
	4m 31s

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