input: Write a function to split a string by a given delimiter.
  assert split_str("hello world, welcome to python", " ") == ["hello", "world,", "welcome", "to", "python"]
  assert split_str("apple-orange-banana", "-") == ["apple", "orange", "banana"]
  assert split_str("one,two,three,four", ",") == ["one", "two", "three", "four"]
output: def split_str(s, delimiter):
      return s.split(delimiter)