Support for pickling sentinel objects as singletons #2723

Job	Run time
Run tests (3.10)	12s
Run tests (3.11)	11s
Run tests (3.11.0)	17s
Run tests (3.9)	17s
Run tests (pypy3.9)	17s
Run tests (3.13)	14s
Run tests (3.13.0)	18s
Run tests (3.12.0)	17s
Run tests (3.9.12)	16s
Run tests (pypy3.10)	18s
Run tests (3.14)	8s
Run tests (3.12)	11s
Run tests (pypy3.11)	27s
Run tests (3.10.4)	16s
Create an issue if daily tests failed	-1s
	3m 38s

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