Use format() instead of an f-string.

To support Python 3.5.

tests/test_vimgolf.py

@@ -59,7 +59,7 @@ def test_show_command(self):
         # Use the challenge with fewest entries to avoid VimGolf Issue #306.
         #   https://github.com/igrigorik/vimgolf/issues/306
         entry = min((x, idx) for idx, x in enumerate(entry_counts))[1] + 1
-        self.assertEqual(main(['vimgolf', 'show', f'+{entry}']), 0)
+        self.assertEqual(main(['vimgolf', 'show', '+{}'.format(entry)]), 0)
         # The following ID is for 'Pascal's Triangle'
         # http://www.vimgolf.com/challenges/5ca2bc786b547e000c77fd52
         challenge_id = '5ca2bc786b547e000c77fd52'