docs: def requirements(requirements_file):
      """Return packages mentioned in the given file.
  
      Args:
          requirements_file (str): path to the requirements file to be parsed.
  
      Returns:
          (list): 3rd-party package dependencies contained in the file.
      """
      return [
          str(pkg.req) for pkg in parse_requirements(
              requirements_file, session=pip_download.PipSession()) if pkg.req is not None]