input: Write a python function to find the longest palindrome in a list of strings.
  assert longest_palindrome(["racecar", "python", "mom", "abba"]) == "racecar"
  assert longest_palindrome(["noon", "civic", "level", "deified"]) == "deified"
  assert longest_palindrome(["a", "bb", "ccc", "dddd"]) == "dddd"
output: def longest_palindrome(words):
      def is_palindrome(word):
          return word == word[::-1]
      
      longest = ""
      for word in words:
          if is_palindrome(word) and len(word) > len(longest):
              longest = word
      return longest