use check_output Python function instead of run when check=True

Consulting the Python docs, I realise I had the misunderstanding that the
`input` parameter is only available for `run`. It is actually also available for
`check_output` as far back as Python 3.4. With that in mind, I have to disagree
with the Python docs:

  The recommended approach to invoking subprocesses is to use the run() function
  for all use cases it can handle.

As soon as you are passing `check=True`, the `check_output` function is both
fewer characters and more backwards compatible.