The test_keywords() test of test_os.TestSendfile emits a warning about dangling threads:

haypo@selma$ ./python -m test --fail-env-changed -R 3:3 test_os -m test.test_os.TestSendfile.test_keywords
Run tests sequentially
0:00:00 load avg: 0.53 [1/1] test_os
Warning -- threading_cleanup() failed to cleanup 0 threads after 3 sec (count: 0, dangling: 2)
beginning 6 repetitions
123456
Warning -- threading_cleanup() failed to cleanup 0 threads after 3 sec (count: 0, dangling: 2)
.Warning -- threading_cleanup() failed to cleanup 0 threads after 3 sec (count: 0, dangling: 2)
.Warning -- threading_cleanup() failed to cleanup 0 threads after 3 sec (count: 0, dangling: 2)
.Warning -- threading_cleanup() failed to cleanup 0 threads after 3 sec (count: 0, dangling: 2)
.Warning -- threading_cleanup() failed to cleanup 0 threads after 3 sec (count: 0, dangling: 2)
.Warning -- threading_cleanup() failed to cleanup 0 threads after 3 sec (count: 0, dangling: 2)
.
test_os failed (env changed)

1 test altered the execution environment:
    test_os

Total duration: 21 sec
Tests result: ENV CHANGED