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/feature_maxuploadtarget.py

@@ -104,7 +104,7 @@ class MaxUploadTest(BitcoinTestFramework):
         assert_equal(len(self.nodes[0].getpeerinfo()), 3)
         # At most a couple more tries should succeed (depending on how long
         # the test has been running so far).
-        for i in range(3):
+        for _ in range(3):
             p2p_conns[0].send_message(getdata_request)
         p2p_conns[0].wait_for_disconnect()
         assert_equal(len(self.nodes[0].getpeerinfo()), 2)