[BUG] Bare connections cannot be used as context managers

• asyncpg version: 0.20.1
• PostgreSQL version: 10.3
• Python version: 3.8.2
• Platform: Windows
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: Yes

While under most use cases, people will want to use connection pools to manage their connections, there are times where using bare connections turns out to be more practical. However, while connection pools can be opened and closed via context managers:

async with asyncpg.create_pool(user='postgres', command_timeout=60) as pool:
    async with pool.acquire() as con:
        await con.fetch('SELECT 1')

...it appears that bare connections created via asyncpg.connect do not support this:

async with asyncpg.connect(user='postgres') as con:
    await con.fetch('SELECT 1')

File "./test.py", line 120, in fetch_ip_list
    async with asyncpg.connect(user="postgres") as con:
AttributeError: __aexit__

I personally like using with blocks to handle the scope of resources like this, so this is a step down for me in terms of ergonomics, not to mention rather surprising given that connections coming from connection pools do work as context managers (even though those are merely returned to the pool instead of closed).

从 asyncpg.connect 入口开始，将其返回的 connection 与 issue 中所示的 pool 和 pool.acquire 上下文管理器行为进行比较。验证 bare connection 预期的 async with 生命周期，并为报告中的示例添加针对性覆盖；当示例进入和退出时不会出现 AttributeError，即表示完成。