TST: use capsys.readouterr() as named tuple
currently only one test in codebase uses capsys.readouterr().

from pytest 3.3, the return value from readouterr changed to a namedtuple with two attributes, out and err.

this change in the style of https://docs.pytest.org/en/latest/capture.html#accessing-captured-output-from-a-test-function is to ensure that the current usage in the codebase does not set a precedent for future usage.