[issue31234] Make support.threading_cleanup() stricter

[STINNER Victor]

STINNER Victor added the comment:

Oh, test_concurrent_futures.test_exception_with_success() failed on Travis CI :-(

https://ci.appveyor.com/project/python/cpython/build/3.7.0a0.6457

test_exception_with_success (test.test_concurrent_futures.FutureTests) ... Warning -- threading_cleanup() failed to cleanup 1 threads (count: 1, dangling: 2)
Dangling thread: <Thread(Thread-1, started 4012)>
Dangling thread: <_MainThread(MainThread, started 3656)>
ok

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<https://bugs.python.org/issue31234>
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