[python] Add back __enter__ and __exit__ methods for non asyncio (#16979)

samples/client/echo_api/python/openapi_client/api_client.py

@@ -90,6 +90,11 @@ class ApiClient:
         self.user_agent = 'OpenAPI-Generator/1.0.0/python'
         self.client_side_validation = configuration.client_side_validation
 
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        pass
 
     @property
     def user_agent(self):