input: Write a function to implement a basic blockchain in Python using only the core modules. The function should include a class `Blockchain` with an initialization method, a method to create a new block, and a method to hash a block. The class should also include a method to return the current timestamp. The function should have the following structure:
  import hashlib
  import json
  import time
  
  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 Blockchain().create_block(12345, "0" * 64) is not None
  assert Blockchain().hash_block({'block_number': 1, 'timestamp': 1234567890, 'nonce': 12345, 'previous_hash': "0" * 64}) == 'b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9'
  assert Blockchain()._current_timestamp() is not None
output: 
  import hashlib
  import json
  import time
  
  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
  