Currently, support.threading_cleanup() waits 1 second before emitting a warning about threads running in the background. I propose to emit immediately the warning to be able to catch bugs quicker, bugs like bpo-31233 or bpo-30830.

The problem is that currently, 12 test files still leak threads:
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12 tests altered the execution environment:
    test_asyncio test_concurrent_futures test_fork1 test_httpservers
    test_logging test_pydoc test_ssl test_thread test_threaded_import
    test_threading test_wait3 test_xmlrpc
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Since threading_cleanup() warnings now mark tests as failed, we should first fix these tests before being able to make support.threading_cleanup() stricter.