vstinner@apu$ ./python -m test test_posix -R 3:3 -m test.test_posix.PosixTester.test_posix_spawn_file_actions
Run tests sequentially
0:00:00 load avg: 0.58 [1/1] test_posix
beginning 6 repetitions
123456
......
test_posix leaked [7, 7, 7] references, sum=21
test_posix leaked [1, 2, 1] memory blocks, sum=4
test_posix failed

1 test failed:
    test_posix

Total duration: 315 ms
Tests result: FAILURE


I bet that the leak was introduced by:

commit 6c6ddf97c402709713d668d0ed53836a7749ba99
Author: Pablo Galindo <Pablogsal@gmail.com>
Date:   Mon Jan 29 01:56:10 2018 +0000

    bpo-20104: Expose `posix_spawn` in the os module (GH-5109)
    
    Add os.posix_spawn to wrap the low level POSIX API of the same name.
    
    Contributed by Pablo Galindo.

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IMHO this issue is easy to fix, so I suggest to leave it to new contributors, or the author of the change.