input: Write a python function to normalize a string by converting it to lowercase and removing special characters.
  assert normalize_string("David!XQ35") == "davidxq35"
  assert normalize_string("Hello, World!") == "helloworld"
  assert normalize_string("Python-3.8") == "python38"
  assert normalize_string("Normalize_THIS!") == "normalizethis"
output: def normalize_string(s):
      return ''.join(c.lower() for c in s if c.isalnum())