Python <= 3.8 is not supported

[PRPA1984]

According to the README.md, the required Python version is >= 3.7. I think this is wrong, because the used type annotations are available in Python >= 3.9.

Here is an example

def prepare(t5: HFEmbedder, clip: HFEmbedder, img: Tensor, prompt: str) -> dict[str, Tensor]:

Return type should be replaced by Dict with its corresponding "from typing import Dict" import

def prepare(t5: HFEmbedder, clip: HFEmbedder, img: Tensor, prompt: str) -> Dict[str, Tensor]:

[ToTheBeginning]

Thanks for you suggestion, we change the dependency to Python >= 3.9 now.