Multihost bug when working with SqlAlchemy

• asyncpg version: 0.24.0
• PostgreSQL version: 10
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?:
• Python version: 3.9.6
• Platform: Doesn't matter
• 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?: doesn't matter

When we use asyncpg via sqlalchemy, we pass a dsn string to sqlalchemy and it parses this string and call asyncpg connect with host, port, username and etc instead of dsn. And it looks like asyncpg can't handle multiple hosts, separated by commas, passed in host param.

首先跟踪 SQLAlchemy 如何解析 DSN 并使用 host 参数调用 asyncpg，然后使用报告中的版本复现包含多个逗号分隔的 PostgreSQL 主机的情况。当 SQLAlchemy 将解析后的 host 值传递给 asyncpg 时，多主机连接行为能够正常工作，即视为完成。