[tests] Recommend f-strings for formatting, update feature_block to use them

test/functional/README.md

@@ -34,7 +34,7 @@ don't have test cases for.
 - When subclassing the BitcoinTestFramework, place overrides for the
   `set_test_params()`, `add_options()` and `setup_xxxx()` methods at the top of
   the subclass, then locally-defined helper methods, then the `run_test()` method.
-- Use `'{}'.format(x)` for string formatting, not `'%s' % x`.
+- Use `f'{x}'` for string formatting in preference to `'{}'.format(x)` or `'%s' % x`.
 
 #### Naming guidelines