Fix flask send_file arguments

The deprecated attachment_filename was removed in a recent flask release
and is superseded by download_name [1].

[1]: https://github.com/pallets/flask/pull/4667

listssrht/blueprints/archives.py

@@ -431,4 +431,4 @@ def export_archive(owner_name, list_name):
     f = open(path, "rb")
     os.unlink(path)
     return send_file(f, as_attachment=True,
-            attachment_filename=f"{owner.username}-{list_name}.mbox")
+            download_name=f"{owner.username}-{list_name}.mbox")

listssrht/blueprints/settings.py

@@ -1,5 +1,5 @@
 from flask import Blueprint, render_template, abort, request, redirect, url_for
-from flask import current_app, send_file, session
+from flask import current_app, session
 from srht.config import cfg
 from srht.database import db
 from srht.flask import paginate_query