I've hit a problem where test_builtin and test_unittest are failing for me when refleak hunting is enabled (as in actual test failures, not just leak reports), but those also appear for me without the patch applied.

Current plan:

- ensure "./python -m test -R 3:3 -x test_builtin test_unittest" is clean both with and without the patch (perhaps also removing some other tests that are unreliable even without the patch)
- file a separate issue for the refleak hunting problem with the error tracebacks
- push the fix for the __classcell__ problems to 3.6