unix_socket: allow_none (needed for traitlets 5.1.0)

Author: manics

jupyter_server_proxy/config.py

@@ -131,6 +131,7 @@ class ServerProcess(Configurable):
     unix_socket = Union(
         [Bool(False), Unicode()],
         default_value=None,
+        allow_none=True,
         help="""
         If set, the service will listen on a Unix socket instead of a TCP port.
         Set to True to use a socket in a new temporary folder, or a string