[python] Issue 13997 fix pass in model instances to new

Created by: spacether

[python] Issue 13997 fix pass in model instances to new This PR is synonymous with https://github.com/openapi-json-schema-tools/openapi-json-schema-generator/pull/82

@Linux13524 With my update, this test code passes when the models from the issue were generated:

ev = event.Event(name="test", properties={"name": "test"})
er = event_request.EventRequest(event=ev)
self.assertEqual(
    er,
    {
        'event': {
            'name': 'test',
            'properties': {'name': 'test'}
        }
    }
)

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