input: Write a function to convert a given string to its lowercase version. Return the new string with all characters converted to lowercase.
  def str_to_lower(string):
      new_string = ""
  
      for char in string:
          new_string += char.lower()
  
      return new_string
  
  assert str_to_lower("HELLO")=="hello"
  assert str_to_lower("This is a TEST")=="this is a test"
  assert str_to_lower("")==""
  assert str_to_lower("123ABC")=="123abc"
  assert str_to_lower("Python3.8")=="python3.8"
output: def str_to_lower(string):
      return string.lower()