Fix error in blockchain.py introduced in merge

qa/rpc-tests/blockchain.py

@@ -80,7 +80,7 @@ class BlockchainTest(BitcoinTestFramework):
         assert isinstance(header['mediantime'], int)
         assert isinstance(header['nonce'], int)
         assert isinstance(header['version'], int)
-        assert isinstance(header['difficulty'], decimal.Decimal)
+        assert isinstance(header['difficulty'], Decimal)
 
 if __name__ == '__main__':
     BlockchainTest().main()