test: Avoid F541 (f-string without any placeholders)

test/functional/feature_dersig.py

@@ -130,7 +130,7 @@ class BIP66Test(BitcoinTestFramework):
         block.hashMerkleRoot = block.calc_merkle_root()
         block.solve()
 
-        with self.nodes[0].assert_debug_log(expected_msgs=[f'Block validation error: mandatory-script-verify-flag-failed (Non-canonical DER signature)']):
+        with self.nodes[0].assert_debug_log(expected_msgs=['Block validation error: mandatory-script-verify-flag-failed (Non-canonical DER signature)']):
             peer.send_and_ping(msg_block(block))
             assert_equal(int(self.nodes[0].getbestblockhash(), 16), tip)
             peer.sync_with_ping()