Reading from https://docs.python.org/3/library/subprocess.html#subprocess.CompletedProcess

"""
If you ran the process with stderr=subprocess.STDOUT, stdout and stderr will be combined in this attribute, and stderr will be None.
"""

But, if you run `run()` with `capture_output=True`, you get the following exception:

"""
ValueError: stdout and stderr arguments may not be used with capture_output.
"""

So, it seems impossible to get the combined outputs of stdout and stderr with `run()`.