Looks like macOS rather than Windows, and I can't reproduce it locally with current master:
14:28 $ ./python.exe -m test -R3:3 test_multiprocessing_fork
Run tests sequentially
0:00:00 load avg: 2.68 [1/1] test_multiprocessing_fork
beginning 6 repetitions
123456
......
test_multiprocessing_fork passed in 9 min 20 sec
1 test OK.
Total duration: 9 min 20 sec
Tests result: SUCCESS
The Windows and Linux refleak buildbots [1] are currently unhappy about other things that appear to be unrelated.
[1] http://buildbot.python.org/all/#/builders?tags=%2Brefleak