test: use f-strings in interface_*.py tests

test/functional/interface_rpc.py

@@ -16,7 +16,7 @@ def expect_http_status(expected_http_status, expected_rpc_code,
                        fcn, *args):
     try:
         fcn(*args)
-        raise AssertionError("Expected RPC error %d, got none" % expected_rpc_code)
+        raise AssertionError(f"Expected RPC error {expected_rpc_code}, got none")
     except JSONRPCException as exc:
         assert_equal(exc.error["code"], expected_rpc_code)
         assert_equal(exc.http_status, expected_http_status)