input: Write a python function to encrypt and decrypt a string using the Fernet module.
  assert encrpyt_string('hello') is not None
  assert decrpyt_string(encrpyt_string('hello')) == 'hello'
  assert decrpyt_string(encrpyt_string('world')) == 'world'
output: from cryptography.fernet import Fernet
  
  # Generate a key
  key = Fernet.generate_key()
  cipher_suite = Fernet(key)
  
  def encrpyt_string(input_string):
      return cipher_suite.encrypt(input_string.encode())
  
  def decrpyt_string(encrypted_string):
      return cipher_suite.decrypt(encrypted_string).decode()