capture_output Isn't a thing in python subprocess module until python3.7

[bstinsonmhk]

In a few places, mbox calls out to the subprocess module and passes the 'capture_output' kwarg. This option wasn't introduced until python 3.7, but the latest we can possibly get on EL7 is python 3.6

We can work around by setting stdout and stderr to subprocess.PIPE

I can do up a patch for this, but I wanted to open an issue for tracking.