Fix for lexing Python raw f-strings with backslashes

[jmd-dk]

Fix for #1681

• Introduce a new 'rfstringescape', used for raw f-strings.
• Build 'fstringescape' as a composite of 'rfstringescape' and 'stringescape'.

[Anteru]

Thanks for preparing the fix! Can you please add an unit test for this as well?

[jmd-dk]

Thanks for preparing the fix! Can you please add an unit test for this as well?

I've added three tests; one for just "rawness", one for just usual f-string behavior and one for the previously missing combined behavior.

[jmd-dk]

@Anteru Note that I've rewritten the old 'fstringescape' in terms of the new 'rfstringescape', as to not have the code

(r'\{\{', String.Escape),
(r'\}\}', String.Escape),

twice. Is this done correctly?

[Anteru]

Sounds reasonable to me, thanks for the contribution!

[birkenfeld]

LGTM.