input: from typing import List
  
  def str_to_lower(string: str) -> str:
      """ Converts a given string to its lowercase version.
      >>> str_to_lower("Hello World")
      'hello world'
      >>> str_to_lower("PYTHON")
      'python'
      >>> str_to_lower("123abc!")
      '123abc!'
      """
      return string.lower()
output:     return string.lower()