[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).

Bắt đầu từ điểm vào asyncpg.connect và so sánh connection được trả về với hành vi của pool và context-manager pool.acquire được thể hiện trong issue. Xác minh vòng đời async with dự kiến đối với một bare connection và bổ sung coverage tập trung cho ví dụ được báo cáo; hoàn tất khi ví dụ có thể đi vào và thoát ra mà không gặp AttributeError.