input: Write a function to encrypt a given plaintext using Fernet encryption and return the encrypted data encoded in base64.
  assert encrypt('hello world') != 'hello world'
  assert isinstance(encrypt('test data'), bytes)
  assert len(encrypt('another test')) > 0
output: def encrypt(plaintext):
      from cryptography.fernet import Fernet
      import base64
  
      key = Fernet.generate_key()
      cipher_suite = Fernet(key)
      encrypted_data = cipher_suite.encrypt(plaintext.encode())
      return base64.b64encode(encrypted_data)