copy_from_query output function receives bytearray

Nobody has claimed this yet.

* **asyncpg version**: 0.25.0
* **PostgreSQL version**: 13.4 (Ubuntu 13.4-1.pgdg21.04+1) + Timescale 2.4.2
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**:
* **Python version**: 3.9.7
* **Platform**: Ubuntu 21.10
* **Do you use pgbouncer?**: No
* **Did you install asyncpg with pip?**: Yes
* **If you built asyncpg locally, which version of Cython did you use?**:
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: Could not try without uvloop because of https://github.com/MagicStack/asyncpg/issues/240

Hi! The [documentation](https://magicstack.github.io/asyncpg/current/api/index.html?highlight=copy_from_query#asyncpg.connection.Connection.copy_from_query) states that a coroutine function passed as `output` argument will receive a `bytes` argument, but when I do this I get `bytearray`. Seems like a bug either with documentation or the code.

Start with the Connection.copy_from_query entry point and its API documentation, which state that the output coroutine receives bytes. Trace the value passed to output and compare it with the documented behavior. Done means confirming whether the code or documentation is incorrect and covering the agreed behavior with a regression test.

Written by the indexing model from the issue text.