input: Write a python function to split a sentence into words.
  assert split_sentence("This is a sentence.")==['This', 'is', 'a', 'sentence.']
  assert split_sentence("Hello World!")==['Hello', 'World!']
  assert split_sentence("Python is awesome") == ['Python', 'is', 'awesome']
output: def split_sentence(sentence):
      return sentence.split()