input: Write a python function to normalize a given string by converting it to lowercase and removing non-alphanumeric characters except spaces.
  assert normalize_string("Hello, World!") == "hello world"
  assert normalize_string("Python 3.8") == "python 38"
  assert normalize_string("Normalize THIS string!") == "normalize this string"
output: def normalize_string(s):
      return ''.join(c.lower() if c.isalnum() or c.isspace() else '' for c in s)