input: Write a function to encrypt and decrypt strings using the AES encryption algorithm. The function should take a plaintext string as input, encrypt it using a pre-defined 16-byte key, and return the encrypted string. Additionally, the function should be able to take the encrypted string and decrypt it back to the original plaintext.
  
  from Crypto.Cipher import AES
  
  key = b'Sixteen byte key'
  
  def encrypt(plaintext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.encrypt(plaintext)
  
  def decrypt(ciphertext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.decrypt(ciphertext)
  
  assert encrypt(b'Hello, World!')
  assert decrypt(encrypt(b'Hello, World!'))==b'Hello, World!'
output: def encrypt(plaintext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.encrypt(plaintext)
  
  def decrypt(ciphertext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.decrypt(ciphertext)