style: use .format() instead of % formatting
diff --git a/test/test_rs485.py b/test/test_rs485.py
index 1d7ed09..153b345 100644
--- a/test/test_rs485.py
+++ b/test/test_rs485.py
@@ -59,7 +59,7 @@
     sys.stdout.write(__doc__)
     if len(sys.argv) > 1:
         PORT = sys.argv[1]
-    sys.stdout.write("Testing port: %r\n" % PORT)
+    sys.stdout.write("Testing port: {!r}\n".format(PORT))
     sys.argv[1:] = ['-v']
     # When this module is executed from the command-line, it runs all its tests
     unittest.main()