PROTON-2149: session state is a property, not a method

This closes #216.
diff --git a/python/proton/_handlers.py b/python/proton/_handlers.py
index 86d083a..443f728 100644
--- a/python/proton/_handlers.py
+++ b/python/proton/_handlers.py
@@ -1082,7 +1082,7 @@
     @staticmethod
     def on_session_remote_open(event):
         ssn = event.session
-        if ssn.state() & Endpoint.LOCAL_UNINIT:
+        if ssn.state & Endpoint.LOCAL_UNINIT:
             ssn.open()
 
     @staticmethod