Support for pickling sentinel objects as singletons #2120

Job	Run time
Lint	9s
Run tests (3.10)	9s
Run tests (3.9)	8s
Run tests (3.9.12)	16s
Run tests (3.11.0)	18s
Run tests (3.11)	9s
Run tests (3.12)	7s
Run tests (3.10.4)	13s
Run tests (3.12.0)	17s
Run tests (3.13)	9s
Run tests (pypy3.9)	9s
Run tests (3.13.0)	14s
Run tests (3.14)	14s
Run tests (pypy3.10)	9s
Create an issue if daily tests failed	0s
	2m 41s

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