apply pyupgrade

examples/tutorial/flaskr/auth.py

@@ -64,7 +64,7 @@ def register():
             db.execute("SELECT id FROM user WHERE username = ?", (username,)).fetchone()
             is not None
         ):
-            error = "User {0} is already registered.".format(username)
+            error = f"User {username} is already registered."
 
         if error is None:
             # the name is available, store it in the database and go to