input: def create_basic_blockchain(self, nonce, previous_hash):
      """
      Create a basic blockchain in Python using only the core modules.
      >>> blockchain = Blockchain()
      >>> initial_block = blockchain.create_block(12345, '0')
      >>> blockchain.hash_block(initial_block)
      '3b7e2e6b8f0b1ff9e15f504f385bc2b6a3c8eb0f5ce8e4fd6e33fa2cc0a9fb3a'
      """
output:     block = {
          'index': len(self.chain) + 1,
          'timestamp': time.time(),
          'nonce': nonce,
          'previous_hash': previous_hash
      }
      self.chain.append(block)
      return block