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 @@ def async_to_sync(
 
     def url_for(
         self,
+        /,
         endpoint: str,
         *,
         _anchor: str | None = None,

tests/test_helpers.py

@@ -161,6 +161,13 @@ def post(self):
         assert flask.url_for("myview", id=42, _method="GET") == "/myview/42"
         assert flask.url_for("myview", _method="POST") == "/myview/create"
 
+    def test_url_for_with_self(self, app, req_ctx):
+        @app.route("/<self>")
+        def index(self):
+            return "42"
+
+        assert flask.url_for("index", self="2") == "/2"
+
 
 def test_redirect_no_app():
     response = flask.redirect("https://localhost", 307)