let typing be compatible with python 3.8

[dlee992]

Hi, when I import prefect_ray with python 3.8, I met a typing error:

→ python -c "import prefect_ray as pr; print(pr.__file__, pr.__version__)"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File ".../.venv/lib/python3.8/site-packages/prefect_ray/__init__.py", line 6, in <module>
    from .task_runners import RayTaskRunner  # noqa
  File ".../.venv/lib/python3.8/site-packages/prefect_ray/task_runners.py", line 91, in <module>
    class RayTaskRunner(BaseTaskRunner):
  File ".../.venv/lib/python3.8/site-packages/prefect_ray/task_runners.py", line 157, in RayTaskRunner
    call: Callable[..., Awaitable[State[R]]],
TypeError: 'ModelMetaclass' object is not subscriptable

So I tried to give a fix about this issue. prefect_dask has the similar issue I think.
Not sure where to add a test for this.

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[desertaxle]

Thanks for the PR @dlee992! I think we'll need to stagger these changes since the change in the core prefect need to be release before we can use them in integrations packages. Can you update this PR to only update the prefect package and then open a separate PR to update prefect-dask and prefect-ray?

[dlee992]

Sure! I can do that.

BTW, need to ask first: does prefect want to accept this PR, given python 3.8 will go to the end of its lifecycle in Oct, 2024. I mean after that time, this PR shouldn't be needed any more, then perhaps incur a revert PR for this.

Perhaps no bother to solve this issue, just put this PR as a temp patch if anyone wants to use prefect_ray/dask with 3.8

[desertaxle]

Great question! We are planning on dropping support for 3.8 in the next few months. We are also planning on updating our task runner interface soon which would resolve this issue.

I'd be happy to accept a PR that fixes the typing in prefect-ray and prefect-dask, but let's scope the changes to only prefect-ray and prefect-dask.