input: Write a function to classify a sentence's sentiment as positive, negative, or neutral.
  assert classify_sentence("I love sunny days!") == 'positive'
  assert classify_sentence("I hate getting up early.") == 'negative'
  assert classify_sentence("It's an average day.") == 'neutral'
output: def classify_sentence(sentence):
      positive_words = ['love', 'happy', 'joy', 'wonderful', 'delightful', 'sunny']
      negative_words = ['hate', 'sad', 'angry', 'terrible', 'awful', 'bad']
      
      sentence = sentence.lower()
      
      if any(word in sentence for word in positive_words):
          return 'positive'
      elif any(word in sentence for word in negative_words):
          return 'negative'
      else:
          return 'neutral'