Simplify code on stdio_client

[Kludex]

This PR still doesn't solve the issue we have: if the process fails to start, it needs to raise. It doesn't. User is seeing Client disconnected message when running client.initialize().

Maybe we can play with something like this in the task group (I'm not sure...):

async def process_cancellation(cancel_scope: anyio.CancelScope):
    _ = await process.wait()
	cancel_scope.cancel()

---

It would be cool for open_process to raise an exception on returncode != None.

[Kludex]

It actually never triggers OSError here. anyio.open_process doesn't trigger.

Maybe the Windows logic does, but even if it does... The streams don't need to be closed because they are not even open yet, so just removing the except is fine.

[Kludex]

According to the test suite I'm wrong... I'm not sure how.

[Kludex]

No need to close read_stream and write_stream, just add the async context manager block above, and no need for read_stream_writer and write_stream_reader because they are open and closed within the tasks above.

[Kludex]

Well, it seems I was wrong... 👀

[Kludex]

Readability changes - I can revert if wanted.