Closes#18671
Note that while this has, I believe, always been invalid syntax, it was
reported as a different syntax error until Python 3.12:
Python 3.11:
```pycon
>>> x = 1
>>> f"{x! s}"
File "<stdin>", line 1
f"{x! s}"
^
SyntaxError: f-string: invalid conversion character: expected 's', 'r', or 'a'
```
Python 3.12:
```pycon
>>> x = 1
>>> f"{x! s}"
File "<stdin>", line 1
f"{x! s}"
^^^
SyntaxError: f-string: conversion type must come right after the exclamanation mark
```
