code: def get_block_header_by_hash(self, block_hash: Hash32) -> BlockHeader:
          
          validate_word(block_hash, title="Block Hash")
          return self.chaindb.get_block_header_by_hash(block_hash)
docs: Returns the requested block header as specified by block hash.
  
          Raises BlockNotFound if there's no block header with the given hash in the db.