[$] Formalizing f-strings

Python's formatted strings, or "f-strings", came relatively late to thelanguage, but have become a popular feature. F-strings allow a compactrepresentation for the common task of interpolating program data intostrings, often in order to output them in some fashion. Somerestrictions were placed on f-strings to simplify the implementation ofthem, but those restrictions are not really needed anymore and, infact, are complicating the CPython parser. That has led to a PythonEnhancement Proposal (PEP) to formalize the syntax of f-strings for thebenefit of Python users while simplifying the maintenance of theinterpreter itself.