bpo-38964: Print correct filename on a SyntaxError in an fstring (GH-20399)

When a `SyntaxError` in the expression part of a fstring is found,
the filename attribute of the `SyntaxError` is always `<fstring>`.
With this commit, it gets changed to always have the name of the file
the fstring resides in.

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>