input: Write a python function to normalize a string by converting it to lowercase and stripping any leading or trailing whitespace.
  assert normalize_string(" Hello World! ") == "hello world!"
  assert normalize_string(" PYTHON  ") == "python"
  assert normalize_string("  Normalize THIS String!  ") == "normalize this string!"
output: def normalize_string(s):
      return s.strip().lower()