Use highest possible pickle protocol

jedi/evaluate/compiled/subprocess/__main__.py

@@ -45,5 +45,7 @@ else:
     load('jedi')
     from jedi.evaluate.compiled import subprocess  # NOQA
 
+# Retrieve the pickle protocol.
+pickle_protocol = int(sys.argv[2])
 # And finally start the client.
-subprocess.Listener().listen()
+subprocess.Listener(pickle_protocol).listen()