Simplify subprocess use with high level run API

Per the Python docs, subprocess.run() is the preferred interface for
running subprocess and replaces the older API (such as
subprocess.check_output())

https://docs.python.org/3/library/subprocess.html

> The recommended approach to invoking subprocesses is to use the run()
> function for all use cases it can handle. For more advanced use cases,
> the underlying Popen interface can be used directly.
>
>
> The run() function was added in Python 3.5; if you need to retain
> compatibility with older versions, see the Older high-level API
> section.

Allows for:

- Return a str from get_output() to decode once instead of every line of
  the result.

- Pass command arguments as a list of strings to ensure no possibility
  for unescaped shell injection.