input: Write a python function to split a string by spaces.
  assert string_split("Hello there, how are you?") == ["Hello", "there,", "how", "are", "you?"]
  assert string_split("This is a test") == ["This", "is", "a", "test"]
  assert string_split("Another example string") == ["Another", "example", "string"]
output: def string_split(s):
      return s.split(' ')