Python PEP 701 f-strings changes

https://peps.python.org/pep-0701/
https://discuss.python.org/t/pep-701-syntactic-formalization-of-f-strings/22046

```python
# https://peps.python.org/pep-0701/#rationale
f"{f"{f"infinite"}"}" + " " + f"{f"nesting!!!"}"
f"{'\n'.join(a)}"

# https://discuss.python.org/t/pep-701-syntactic-formalization-of-f-strings/22046/93
f"___{
    x
}___"
```