input: Write a function to check if a string contains only alphanumeric characters.
  assert is_alphanumeric("Hello123") == True
  assert is_alphanumeric("Hello 123") == False
  assert is_alphanumeric("Hello@123") == False
  assert is_alphanumeric("123456") == True
output: def is_alphanumeric(s):
      return s.isalnum()