Properly access ocpp version enum value

Fixes central_system fixture for python 3.11

Signed-off-by: Kai-Uwe Hermann <[email protected]>

everest-testing/src/everest/testing/ocpp_utils/central_system.py

@@ -124,7 +124,7 @@ async def start(self, ssl_context=None):
             self.on_connect,
             '0.0.0.0',
             self.port,
-            subprotocols=[self.ocpp_version],
+            subprotocols=[self.ocpp_version.value],
             ssl=ssl_context
         )
         if self.port is None: