Cursor cannot be used with externally-managed transaction

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Asyncpg 0.31.0, installed from pip and running against a local pg database. This is not a deployment issue, but rather an issue w/the check performed by `Cursor._check_ready()`.

https://github.com/MagicStack/asyncpg/blob/db8ecc2a38e16fb0c090aef6f5506547c2831c24/asyncpg/cursor.py#L114-L116

Working on implementing streaming results for Peewee and I'm running into an issue w/the Cursor `_check_ready()`. Currently it requires an asyncpg-managed transaction in order to proceed. Is there a reason it doesn't do a similar check to the one here, where it queries `is_in_transaction()`?

https://github.com/MagicStack/asyncpg/blob/db8ecc2a38e16fb0c090aef6f5506547c2831c24/asyncpg/connection.py#L1532

I'd like to be able to use server-side cursors with Peewee's library-managed transactions.

Start with asyncpg/cursor.py at Cursor._check_ready(), then compare its transaction check with connection.py at is_in_transaction(). Determine whether readiness can recognize an externally managed transaction without breaking asyncpg-managed cursors; done means server-side cursors work with Peewee's library-managed transactions.

Written by the indexing model from the issue text.