Support for pickling sentinel objects as singletons #2084

Job	Run time
Lint	6s
Run tests (3.9)	7s
Run tests (3.9.12)	11s
Run tests (3.10)	8s
Run tests (3.10.4)	15s
Run tests (3.11)	5s
Run tests (3.11.0)	14s
Run tests (3.12)	7s
Run tests (3.12.0)	14s
Run tests (3.13)	5s
Run tests (3.13.0)	16s
Run tests (3.14)	18s
Run tests (pypy3.9)	9s
Run tests (pypy3.10)	7s
Create an issue if daily tests failed	0s
	2m 22s

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