The first message was on Python 3.6. On master, regrtest sees even more kinds of leaks: references, memory blocks and file descriptors.

haypo@selma$ ./python -m test --match=test.test_multiprocessing_forkserver.WithProcessesTestBarrier.test_barrier --match=test.test_multiprocessing_forkserver.WithProcessesTestFinalize.test_finalize -R 3:3 test_multiprocessing_forkserver 
Run tests sequentially
0:00:00 load avg: 0.25 [1/1] test_multiprocessing_forkserver
beginning 6 repetitions
123456
......
test_multiprocessing_forkserver leaked [492, 492, 492] references, sum=1476
test_multiprocessing_forkserver leaked [221, 222, 225] memory blocks, sum=668
test_multiprocessing_forkserver leaked [4, 4, 4] file descriptors, sum=12
test_multiprocessing_forkserver failed

1 test failed:
    test_multiprocessing_forkserver

Total duration: 5 sec
Tests result: FAILURE