input: Write a function to convert a string to its lowercase version. If the input string is empty, return an empty string as well.
  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 PROGRAMMING")=="python programming"
  assert str_to_lower("123!@#aBcDeF")=="123!@#abcdef"
  assert str_to_lower("")==""
output: def str_to_lower(string):
      return string.lower()