Fixes#3506
We can't simply escape the quotes in a naked f-string when merging string groups, because backslashes are invalid.
The quotes in f-string expressions should be toggled (this is safe since quotes can't be reused).
This fix also means implicitly concatenated f-strings with different quotes can now be merged or quote-normalized by changing the quotes used in expressions. e.g.:
```diff
raise sa_exc.UnboundExecutionError(
"Could not locate a bind configured on "
- f'{", ".join(context)} or this Session.'
+ f"{', '.join(context)} or this Session."
)
```
a36878eb2f