input: Write a python function to remove unwanted characters from a string, retaining only alphanumeric characters and spaces.
  assert remove_unwanted_chars("Hello, World!") == "Hello World"
  assert remove_unwanted_chars("Python@3.9") == "Python39"
  assert remove_unwanted_chars("Keep it 100%!") == "Keep it 100"
output: def remove_unwanted_chars(s):
      return ''.join(c for c in s if c.isalnum() or c.isspace())