Run command with arguments and return its output as a byte string.
  Backported from Python 2.7 as it's implemented as pure python on stdlib.

def CheckOutput(*popenargs, **kwargs):
  """
  Run command with arguments and return its output as a byte string.
  Backported from Python 2.7 as it's implemented as pure python on stdlib.
  """
  process = subprocess.Popen(stdout=subprocess.PIPE, *popenargs, **kwargs)
  output, _ = process.communicate()
  retcode = process.poll()
  if retcode:
      cmd = kwargs.get("args")
      if cmd is None:
          cmd = popenargs[0]
      error = subprocess.CalledProcessError(retcode, cmd)
      error.output = output
      raise error
  return retcode, output