[3.9] bpo-38964: Print correct filename on a SyntaxError in an fstring (GH-20399...

[3.9] bpo-38964: Print correct filename on a SyntaxError in an fstring (GH-20399) (GH-20404)

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>.
(cherry picked from commit f7b1e461567e5e3fa3ba46f589d9edc1b45b2dd0)