Release the GIL before block_on

[gatesn]

Fixes #4895

We were deadlocking when calling back into Python's parquet reader because we held the GIL within our Tokio runtime.
We should probably follow up with a pass to release the GIL in more places.

[AdamGS]

I think it'll be nice to have some block_on function that does it and takes Python, forcing callers to do it correctly. You can also limit the exposure of the runtime which will make it more obvious.

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[gatesn]

Because we use into_array_iter and BlockingRuntime, it's not that easy. It can be quite abstract where the release is. We also want to release the GIL for non-block_on functions too, so not sure it's worth wrapping up