input: Write a function to create a basic blockchain using Python's core modules. The function should define a class `Blockchain` with methods to initialize the blockchain, create a new block, get the current timestamp, and hash a block. The output should be an instance of the `Blockchain` class with a chain of blocks.
  assert create_blockchain().chain == []
  assert len(create_blockchain().create_block(nonce=12345, previous_hash='0').keys()) == 4
  assert type(create_blockchain()._current_timestamp()) == int
  assert len(create_blockchain().hash_block({'block_number': 1, 'timestamp': 1234567890, 'nonce': 12345, 'previous_hash': '0'})) == 64
output: class Blockchain:
      def __init__(self):
          self.chain = []
  
      def create_block(self, nonce, previous_hash):
          block = {
              'block_number': len(self.chain) + 1,
              'timestamp': self._current_timestamp(),
              'nonce': nonce,
              'previous_hash': previous_hash
          }
          self.chain.append(block)
          return block
  
      def _current_timestamp(self):
          return int(time.time())
  
      def hash_block(self, block):
          block_string = json.dumps(block, sort_keys=True).encode()
          return hashlib.sha256(block_string).hexdigest()