> I don't think that it's worth it to backport the change to 3.7 and 3.8: in Python 3.7 and 3.8, the code is accepted.

> Python 3.7 and 3.8 are not broken, so there is no need to fix them :-)

I think we could very reasonably change  `else"#fca"` -> `else "#fca"` on the current bugfix branches, or at least 3.8. Even when it doesn't cause a syntax error and the attempted prefix is ignored, it's still more syntactically correct compared to attempting to use an "e-string" (which of course doesn't exist) and has nearly zero cost on our end to fix. 

While it's a low priority issue since it doesn't raise an error, that doesn't necessarily mean it's correct. In the latest stable version of the stdlib, we should aim to provide a good example IMO, especially if it comes at practically zero cost to us.