fix relative input noise test

src/tests/test_plots/test_frequency_domain/test_relative_input_noise.py

@@ -13,7 +13,7 @@ datadir = filedir / "data"
 
 class TestRelativeInputNoise(unittest.TestCase):
     def setUp(self):
-        files = [datadir / "flipper{i}.TXT" for i in range(1, 4)]
+        files = [datadir / f"flipper{i}.TXT" for i in range(1, 4)]
         self.data = io.read(files)
 
     def test_default_values(self):