input: Write a python function to get unique words from a string.
  assert get_unique_words("This is a test") == ['this', 'is', 'a', 'test']
  assert get_unique_words("Hello hello world") == ['hello', 'world']
  assert get_unique_words("Python is great and Python is fun") == ['python', 'is', 'great', 'and', 'fun']
output: def get_unique_words(s):
      words = s.lower().split()
      unique_words = list(dict.fromkeys(words))
      return unique_words