I'm not very familiar with the Smartsheet Python SDK, but in general, I often find it helpful to look at the integration test code in the SDK's GitHub repo whenever I have a question about how to implement a certain operation using the SDK. In this case, the integration test for webhooks in the SDK repo shows the following code for creating a webhook:
def test_create_webhook(self, smart_setup):
smart = smart_setup['smart']
webhook = smart.models.Webhook()
webhook.name = 'My Webhook'
webhook.callback_url = 'https://www.smartsheet.com'
webhook.scope = 'sheet'
webhook.scope_object_id = smart_setup['sheet'].id
webhook.events.append('*.*')
webhook.version = 1
webhook.subscope = smart.models.WebhookSubscope({'column_ids': [smart_setup['sheet'].columns[0].id]})
action = smart.Webhooks.create_webhook(webhook)
assert action.message == 'SUCCESS'
assert isinstance(action.result, smart.models.Webhook)
TestWebhooks.webhook = action.result
Notice specifically that the way the subscope property is being set differs from what you've posted above. I'd suggest trying to model your code more after this code, to see if you can achieve the desired outcome.