Allow self as an argument to url_for

This makes the Flask.url_for self argument positional only (Flask
supports Python 3.8+) thereby restoring the ability to pass self as a
value argument to url_for.

src/flask/app.py

@@ -952,6 +952,7 @@ class Flask(App):
 
     def url_for(
         self,
+        /,
         endpoint: str,
         *,
         _anchor: str | None = None,