Support for pickling sentinel objects as singletons #2083

Job	Run time
Lint	9s
Run tests (3.9)	8s
Run tests (3.9.12)	12s
Run tests (3.10)	7s
Run tests (3.10.4)	15s
Run tests (3.11)	6s
Run tests (3.11.0)	12s
Run tests (3.12)	6s
Run tests (3.12.0)	14s
Run tests (3.13)	8s
Run tests (3.13.0)	12s
Run tests (3.14)	14s
Run tests (pypy3.9)	11s
Run tests (pypy3.10)	6s
Create an issue if daily tests failed	0s
	2m 20s

Provide feedback