compatibility with old subprocess.run API

[mickaelseznec]

Hi, thanks for your nice job :) It just wanted to add this fix this so stui can run on my cluster. Let me know if you want anything changed.

Commit message: subprocess.run has added support for capture_output only in python 3.7. Even if python <=3.6 is officially out of support, users can still encounter it on Slurm frontends, where stui is likely to be used.