input: Write a function to classify a sentence as positive or negative based on the presence of negative words.
  assert classify_sentence('I am happy!') == 'positive'
  assert classify_sentence('I will never give up.') == 'negative'
  assert classify_sentence('This is not good.') == 'negative'
  assert classify_sentence('Everything is fine.') == 'positive'
output: def classify_sentence(sentence):
      negative_words = {'not', 'never', 'no', 'nothing', 'none', 'nobody', 'neither', 'nowhere', 'hardly', 'scarcely', 'barely'}
      words = set(sentence.lower().split())
      if words & negative_words:
          return 'negative'
      return 'positive'