Fix blueprint self registration

By raising a ValueError if attempted. I don't see a use case that
makes this worth supporting.

Author: pgjones

src/flask/blueprints.py

@@ -260,6 +260,8 @@ def register_blueprint(self, blueprint: "Blueprint", **options: t.Any) -> None:
 
         .. versionadded:: 2.0
         """
+        if blueprint is self:
+            raise ValueError("Cannot register a blueprint on itself")
         self._blueprints.append((blueprint, options))
 
     def register(self, app: "Flask", options: dict) -> None:

tests/test_blueprints.py

@@ -883,3 +883,9 @@ def test_unique_blueprint_names(app, client) -> None:
         app.register_blueprint(bp2)  # different bp, same name, error
 
     app.register_blueprint(bp2, name="alt")  # different bp, different name, ok
+
+
+def test_self_registration(app, client) -> None:
+    bp = flask.Blueprint("bp", __name__)
+    with pytest.raises(ValueError):
+        bp.register_blueprint(bp)