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/rpc_psbt.py

@@ -430,7 +430,7 @@ class PSBTTest(BitcoinTestFramework):
         # Check that joining shuffles the inputs and outputs
         # 10 attempts should be enough to get a shuffled join
         shuffled = False
-        for i in range(0, 10):
+        for _ in range(10):
             shuffled_joined = self.nodes[0].joinpsbts([psbt, psbt2])
             shuffled |= joined != shuffled_joined
             if shuffled: