refactor: test: use _ variable for unused loop counters

substitutes "for x in range(N):" by "for _ in range(N):"
indicates to the reader that a block is just repeated N times, and
that the loop counter is not used in the body

test/functional/mempool_updatefromblock.py

@@ -73,7 +73,7 @@ class MempoolUpdateFromBlockTest(BitcoinTestFramework):
                 n_outputs = size - tx_count
                 output_value = ((inputs_value - fee) / Decimal(n_outputs)).quantize(Decimal('0.00000001'))
                 outputs = {}
-                for n in range(0, n_outputs):
+                for _ in range(n_outputs):
                     outputs[self.nodes[0].getnewaddress()] = output_value
             else:
                 output_value = (inputs_value - fee).quantize(Decimal('0.00000001'))