input: Write a function to implement a basic blockchain in Python using only the core modules. The function should define a class called `Blockchain` with methods to create a block, get the current timestamp, and hash a block. The block should include a block number, timestamp, nonce, and previous hash. The function should return the created block after appending it to the chain.
  assert Blockchain().create_block(12345, '0'*64)['block_number'] == 1
  assert Blockchain().create_block(67890, '1'*64)['nonce'] == 67890
  assert len(Blockchain().hash_block(Blockchain().create_block(11111, '2'*64))) == 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.get_current_timestamp(),
              'nonce': nonce,
              'previous_hash': previous_hash
          }
          self.chain.append(block)
          return block
  
      def get_current_timestamp(self):
          import time
          return time.time()
  
      def hash_block(self, block):
          import hashlib
          block_string = str(block).encode()
          return hashlib.sha256(block_string).hexdigest()