input: Write a function to create a basic blockchain in Python using only the core modules. The function should define a class named `Blockchain` with an `__init__` method that initializes an empty chain and transactions list. It should also include a method to create a new block, a method to get the current timestamp, and a method to hash a block.
  
  class Blockchain:
  
   def __init__(self):
   self.chain = []
   self.transactions = []
  
   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 round(time.time() * 1000)
  
   def hash_block(self, block):
   string_object = json.dumps(block, sort_keys=True)
   block_string = string_object.encode()
   raw_hash = hashlib.sha256(block_string)
   hex_hash = raw_hash.hexdigest()
   return hex_hash
  
  assert type(Blockchain()) == Blockchain
  blockchain = Blockchain()
  block = blockchain.create_block(12345, 'abc123')
  assert 'block_number' in block
  assert 'timestamp' in block
  assert 'nonce' in block
  assert 'previous_hash' in block
  assert blockchain.hash_block(block).startswith('0') == False
  assert len(blockchain.chain) == 1
output: 
  def __init__(self):
      self.chain = []
      self.transactions = []
  
  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 round(time.time() * 1000)
  
  def hash_block(self, block):
      string_object = json.dumps(block, sort_keys=True)
      block_string = string_object.encode()
      raw_hash = hashlib.sha256(block_string)
      hex_hash = raw_hash.hexdigest()
      return hex_hash
  