My *guess* is that you created the branch for PR 7491 when your local master branch was months out of date.  Or you branched off of the 3.7 branch.  Whatever, your patch proposed to revert recent changes to master, touching about 1000 files.  When you pushed the button on the github web page to compare and possibly make a PR, you should have seen a list of +- 1000 changes in the proposed PR, closed the window, deleted the branch on the fork and your repository, and started over with a new branch from freshly updated master.  You are right that the review requests were automatic.  However, I am not sure how the 6 month delay factors into what happened.

Since Guido closed the PR rather than suggest fixing it further, I suggest that you start over as suggested above and look closely before pressing 'Create PR'.  It appears that you already deleted the branch from the fork.

From my limited knowledge of how to use unitest.mock correctly, I agree that the example looks plausible.