test: use f-strings in feature_segwit.py

Author: fanquake

test/functional/feature_segwit.py

@@ -65,7 +65,7 @@ def find_spendable_utxo(node, min_value):
         if utxo['spendable']:
             return utxo
 
-    raise AssertionError("Unspent output equal or higher than %s not found" % min_value)
+    raise AssertionError(f"Unspent output equal or higher than {min_value} not found")
 
 txs_mined = {} # txindex from txid to blockhash