input: Write a python function to parse a string by a given separator.
  assert parse_str("apple,banana,cherry", ",") == ["apple", "banana", "cherry"]
  assert parse_str("1-2-3-4-5", "-") == ["1", "2", "3", "4", "5"]
  assert parse_str("hello world", " ") == ["hello", "world"]
output: def parse_str(s, sep):
      return s.split(sep)