input: Write a function to create a basic blockchain in Python using only the core modules. The function should define a class called `Blockchain` with methods to initialize the blockchain, create a new block, and hash a block. The function should also include a method to return the current timestamp.
  assert Blockchain().create_block(0, '0')['block_number'] == 1
  assert Blockchain().hash_block({'block_number': 1, 'timestamp': 1627300000000, 'nonce': 0, 'previous_hash': '0'}) == '3f5a00f3e1e7a9b8f45f38525d2c332831ba0f163bfef0208c9bcf18f3f04a65'
  assert Blockchain()._current_timestamp() > 0
  assert len(Blockchain().chain) == 0
output: class Blockchain:
      def __init__(self):
          self.chain = []
          self.current_block = None
  
      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.current_block = block
          self.chain.append(block)
          return block
  
      def hash_block(self, block):
          import hashlib
          block_string = str(block).encode()
          return hashlib.sha256(block_string).hexdigest()
  
      def _current_timestamp(self):
          import time
          return int(time.time() * 1000)