[BUG] Bare connections cannot be used as context managers

まだ誰も着手していません。

* **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:

```py
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:

```py
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).

asyncpg.connect のエントリーポイントから始め、issue に示されている pool と pool.acquire のコンテキストマネージャーの動作を、返された接続と比較します。単独の接続に対して期待される async with のライフサイクルを検証し、報告された例を対象とする集中的なカバレッジを追加します。例が AttributeError なしで開始・終了できれば完了です。

索引モデルが issue の本文から書いたものです。