f-strings do not show warnings about invalid escapes

[pablogsal]

In 3.12 and 3.13, f-strings are not warning about invalid escapes that get warnings in real strings. This seems like a bug.

>>> f'\?'
'\\?'
>>> len(f'\?')
2
>>> '\?'
<stdin>:1: SyntaxWarning: invalid escape sequence '\?'
'\\?'

_Originally posted by @terryjreedy in https://github.com/python/cpython/issues/105784#issuecomment-1591828755_
            

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### Linked PRs
* gh-105801
* gh-105806
<!-- /gh-linked-prs -->

[pablogsal]

CC: @terryjreedy