input: Write a function that takes a string as input and returns the string converted to its lowercase version. 
  def str_to_lower(string):
      new_string = ""
      for char in string:
          new_string += char.lower()
      return new_string
  
  assert str_to_lower("HelloWorld") == "helloworld"
  assert str_to_lower("PYTHON") == "python"
  assert str_to_lower("Testing Is Great!") == "testing is great!"
  assert str_to_lower("") == ""
  assert str_to_lower("!@#$%^&*()_+") == "!@#$%^&*()_+"
output: def str_to_lower(string):
      return string.lower()