I'm not sure about backporting the new timeout constants of test.support to Python 3.7 and 3.8. Backporting all changes may help to make tests more stable in 3.7 and 3.7 branches. It should also help to backport test changes from master to 3.7 and 3.8. But it's somehow a new feature and so I suggest to not backport yet.

I may backport later if they are too many conflicts or if tests fail too often because of too short timeouts.

I close the issue.