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_persist.py

@@ -62,7 +62,7 @@ class MempoolPersistTest(BitcoinTestFramework):
     def run_test(self):
         self.log.debug("Send 5 transactions from node2 (to its own address)")
         tx_creation_time_lower = int(time.time())
-        for i in range(5):
+        for _ in range(5):
             last_txid = self.nodes[2].sendtoaddress(self.nodes[2].getnewaddress(), Decimal("10"))
         node2_balance = self.nodes[2].getbalance()
         self.sync_all()