Converts a single-page pdf to a png image via the `sips` command
  :returns: Path to the converted png

def to_png(pdf_path):
  ''' Converts a single-page pdf to a png image via the `sips` command
  :returns: Path to the converted png
  '''
  try:
    with NamedTemporaryFile(prefix='pyglass', suffix='.png', delete=False) as tempfileobj:
      png_path = tempfileobj.name

    cmd = ['sips', '-s', 'format', 'png', pdf_path, '--out', png_path]
    assert(check_call(cmd) == 0)

    assert(exists(png_path))
    return png_path
  except:
    return None