Refactor OpenAIEngine to use the openai-python package

[zhudotexe]

In the current version of Kani we don't use the official OpenAI package, instead calling the API ourselves with an aiohttp client. I chose to do this because in the earlier 0.x versions, the OpenAI package did not yet support async HTTP requests and had a weird module-level structure. Since the release of their 1.x version, these concerns are no longer valid, and we should move to using the official package to decrease code fragility.