Message 367658 - Python tracker

From earlier in this issue: https://bugs.python.org/msg366164

> So a slightly shorter example uses ru''. This is an error because you can't combine the r prefix and the u prefix (in fact you can't combine anything with the r prefix).

That's not true: 'rf' is a valid prefix: rf'{1}'. And so are all of the wacky combinations rF, Rf, RF, fr, fR, Fr, FR.