Support Python3.8 for llama-index-packs

[seyeong-han]

To integrate our code into the llama-index, we need to support Python3.8, which was not our option.

The main fix points will be adding manual typing, which was first implemented in Python 3.9.

[kevinl424]

I see, I thought that typing was introduced in Python 3.5? What do you mean by manual typing here?