Don't use `capture_output` param with old version of Python.

[cpb-]

The patch was checked with Python 3.6 on Centos 8 but we can't add an automatic test.

[pboettch]

No, sorry. There has to be a better way to do it. Let me check.

[pboettch]

Maybe we can use subprocess.check_output() instead of run(). https://docs.python.org/3/library/subprocess.html#subprocess.check_output