pytest-subtests prevents stdio_mgr from capturing stdio

[bskinn]

Broken: https://travis-ci.org/bskinn/sphobjinv/jobs/641379020?utm_medium=notification&utm_source=github_status

Fixed: https://travis-ci.org/bskinn/sphobjinv/jobs/641395845?utm_medium=notification&utm_source=github_status