input: Write a python function to find the longest word in a sentence.
  assert longest_word("The quick brown fox jumps over the lazy dog.") == "jumps"
  assert longest_word("A journey of a thousand miles begins with a single step.") == "thousand"
  assert longest_word("To be or not to be, that is the question.") == "question"
output: def longest_word(sentence):
      words = sentence.split()
      longest = max(words, key=len)
      return longest