copy_from_query output function receives bytearray

• 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?: Could not try without uvloop because of https://github.com/MagicStack/asyncpg/issues/240

Hi! The documentation 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.

从 Connection.copy_from_query 入口点及其 API 文档开始，其中说明输出协程接收 bytes。跟踪传递给 output 的值，并将其与文档所述行为进行比较。确认代码或文档哪一方不正确，并通过回归测试覆盖约定的行为，即视为完成。