Support for pickling sentinel objects as singletons #2205

Job	Run time
Run tests (3.10)	10s
Run tests (3.9)	6s
Run tests (3.9.12)	16s
Run tests (3.10.4)	15s
Run tests (3.11)	12s
Run tests (3.11.0)	18s
Run tests (3.12.0)	13s
Run tests (3.12)	6s
Run tests (3.13)	12s
Run tests (pypy3.10)	10s
Run tests (pypy3.11)	20s
Run tests (3.13.0)	18s
Run tests (pypy3.9)	12s
Run tests (3.14)	18s
Create an issue if daily tests failed	0s
	3m 6s

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