Support for pickling sentinel objects as singletons #2090

Job	Run time
Lint	1s
Run tests (3.9)	1s
Run tests (3.9.12)	13s
Run tests (3.10)	1s
Run tests (3.10.4)	1s
Run tests (3.11)	7s
Run tests (3.11.0)	13s
Run tests (3.12)	8s
Run tests (3.12.0)	17s
Run tests (3.13)	10s
Run tests (3.13.0)	13s
Run tests (3.14)	1s
Run tests (pypy3.9)	1s
Run tests (pypy3.10)	1s
Create an issue if daily tests failed	0s
	1m 28s

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