input: Write a python function to create a block and verify its hash using the Blockchain class.
  assert blockchain.create_block(1, '0')['block_number'] == 1
  assert blockchain.hash_block(blockchain.chain[0]) == hashlib.sha256(json.dumps(blockchain.chain[0], sort_keys=True).encode()).hexdigest()
  assert len(blockchain.chain) == 1
output: import hashlib
  import json
  
  class Blockchain:
      def __init__(self):
          self.chain = []
  
      def create_block(self, block_number, previous_hash):
          block = {
              'block_number': block_number,
              'previous_hash': previous_hash
          }
          self.chain.append(block)
          return block
  
      def hash_block(self, block):
          block_string = json.dumps(block, sort_keys=True).encode()
          return hashlib.sha256(block_string).hexdigest()