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To follow up on David's comment, the unit tests in the test suite aren't consistent in their treatment of temp directories (e.g. they don't use a common API or code path). So it may be hard to address this globally short of wiping the entire temp directory (though I could be wrong).
I have a patch in issue 15415 to add a temp_dir() context manager to test.support (and consolidate with script_helper's) that would do such clean-up. The module already has a temp_cwd(), but that context manager serves two purposes.