code: def get(self, block_number: BlockNumber) -> str:
          
          if block_number in self.mapping:
              return self.mapping[block_number]
  
          block_hash = self.web3.eth.getBlock(block_number)['hash']
          block_hash = block_hash.hex()
          self.mapping[block_number] = block_hash
          return block_hash
docs: Given a block number returns the hex representation of the blockhash