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/wallet_basic.py

@@ -569,7 +569,7 @@ class WalletTest(BitcoinTestFramework):
         # So we should be able to generate exactly chainlimit txs for each original output
         sending_addr = self.nodes[1].getnewaddress()
         txid_list = []
-        for i in range(chainlimit * 2):
+        for _ in range(chainlimit * 2):
             txid_list.append(self.nodes[0].sendtoaddress(sending_addr, Decimal('0.0001')))
         assert_equal(self.nodes[0].getmempoolinfo()['size'], chainlimit * 2)
         assert_equal(len(txid_list), chainlimit * 2)