https://docs.python.org/3/library/subprocess.html#subprocess.Popen.returncode

does mention the negativeness about returncode in conjunction with POSIX signals. It would be helpful to mention this Python-specific (I think) behavior in a comment. 
However, returncode can also be set by poll() and wait(), which can also return errors. In the more realistic example, I would appreciate code that handles those errors as well (a check for returncode > 0), or a comment detailing that other errors aren't handled. I copied that code into mine, and didn't consider those cases initially, because "realistic" was in the description.