input: Write a function to convert a string to its lowercase version. If the given string is empty then return an empty string.
  def str_to_lower(string):
      new_string = ""
      
      for char in string:
          new_string += char.lower()
      
      return new_string
  
  assert str_to_lower("Hello, World!")=="hello, world!"
  assert str_to_lower("PYTHON")=="python"
  assert str_to_lower("12345")=="12345"
  assert str_to_lower("")==""
  assert str_to_lower("ALL CAPS")=="all caps"
output: def str_to_lower(string):
      return string.lower()