input: Write a function to split a sentence into words.
  assert sentence_to_words("This is a test sentence.") == ["This", "is", "a", "test", "sentence."]
  assert sentence_to_words("Hello world!") == ["Hello", "world!"]
  assert sentence_to_words("Python is great.") == ["Python", "is", "great."]
output: def sentence_to_words(sentence):
      return sentence.split()