fix __exit__ method of client and server class

opcua/client/client.py

@@ -102,7 +102,7 @@ class Client(object):
         self.connect()
         return self
 
-    def __exit__(self):
+    def __exit__(self, exc_type, exc_value, traceback):
         self.disconnect()
 
     @staticmethod

opcua/server/server.py

@@ -94,7 +94,7 @@ class Server(object):
         self.start()
         return self
 
-    def __exit__(self):
+    def __exit__(self, exc_type, exc_value, traceback):
         self.stop()
 
     def load_certificate(self, path):