Solve UnboundLocalError in launch_browser()

Python's scoping rules cause the log statement to fail
because `_` was being redefined in `launch_browser()`.

Author: jamesmishra

jupyter_server/serverapp.py

@@ -2023,7 +2023,7 @@ def launch_browser(self):
         if not browser:
             return
 
-        assembled_url, _ = self._prepare_browser_open()
+        assembled_url, __ = self._prepare_browser_open()
 
         b = lambda: browser.open(assembled_url, new=self.webbrowser_open_new)
         threading.Thread(target=b).start()