Fix `copy`, `deepcopy` and `pickle` of `Sentinel` members
The story of the bug "Fix `copy`, `deepcopy` and `pickle` of `Sentinel` members" and how each model did on it.
Models fixed it in 6 of 6 recorded runs. The bug is in commit 420c8fb of the repository's git history and the real fix is commit f58ca3e. A model gets the code at the first commit and the repository's own tests decide if it fixed the bug.