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

This isn't specific to test_keywords. Dangling threads occurred even if replace the test body with just "return". This is caused by setUp/tearDown or setUpClass/tearDownClass.

> This isn't specific to test_keywords. Dangling threads occurred even if replace the test body with just "return". This is caused by setUp/tearDown or setUpClass/tearDownClass.
Right, see my PR #2680. The general problem is that tearDownClass() seems to be called while the test case instance is still alive and so didn't clear its attributes. But I wrote a single line fix instead of trying to fix unittest or regrtest :-)

New changeset d1cc037d1442cc35d1b194ec8e50901514360949 by Victor Stinner in branch 'master':
bpo-30908: Fix dangling thread in test_os.TestSendfile (#2680)
https://github.com/python/cpython/commit/d1cc037d1442cc35d1b194ec8e50901514360949

The alternate solution is to remove all references to test instances before calling tearDownClass().
Interesting, that this warning is occurred only in the leak hunting mode. In normal cases test instances are removed from a testsuite, but regrtest hacks unittest and disables this in the leak hunting mode (see the line "unittest.BaseTestSuite._cleanup = False" in Lib/test/libregrtest/setup.py). See issue11798.

Serhiy Storchaka: If you would like to enhance the unittest module, please open a new issue. I prefer to keep this one for the very specific test_os bug.

New changeset 157b6ad677b7b330d30b2bb5ffbb2edac77b78cb by Victor Stinner in branch '3.5':
bpo-30908: Fix dangling thread in test_os.TestSendfile (#2680) (#2845)
https://github.com/python/cpython/commit/157b6ad677b7b330d30b2bb5ffbb2edac77b78cb

New changeset bb33ccfc5a216eff753b10a8dc59ec0277f1e1fb by Victor Stinner in branch '3.6':
bpo-30908: Fix dangling thread in test_os.TestSendfile (#2680) (#2844)
https://github.com/python/cpython/commit/bb33ccfc5a216eff753b10a8dc59ec0277f1e1fb