[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).

從 asyncpg.connect 入口開始，將其傳回的 connection 與 issue 中所示的 pool 和 pool.acquire context-manager 行為進行比較。驗證 bare connection 預期的 async with 生命週期，並為回報的範例新增針對性的 coverage；當範例進入和結束時不會發生 AttributeError，即表示完成。