decode bytes from secure cookie (#562)

The get_secure_cookie interface returns bytes not str

https://www.tornadoweb.org/en/stable/web.html#tornado.web.RequestHandler.get_secure_cookie

This fixes a minor issue where `handler.get_current_user` would return
bytes rather than str.

Author: oliver-sanders

jupyter_server/auth/login.py

@@ -172,6 +172,8 @@ def get_user(cls, handler):
         if user_id is None:
             get_secure_cookie_kwargs  = handler.settings.get('get_secure_cookie_kwargs', {})
             user_id = handler.get_secure_cookie(handler.cookie_name, **get_secure_cookie_kwargs )
+            if user_id:
+                user_id = user_id.decode()
         else:
             cls.set_login_cookie(handler, user_id)
             # Record that the current request has been authenticated with a token.