docs: def load_yaml_file(yaml_file):
      """Read YAML file."""
      try:
          import yaml
      except ImportError:
          sys.exit("Unable to import yaml module.")
      try:
          with io.open(yaml_file, "rt", encoding="utf-8") as fname:
              return yaml.safe_load(fname)
      except IOError:
          sys.exit("Unable to open YAML file: {0}".format(yaml_file))