Fix an invalid quote escaping bug in f-string expressions (#3509)

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."
         )
```