[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:

```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).

Start at the asyncpg.connect entry point and compare its returned connection with the pool and pool.acquire context-manager behavior shown in the issue. Verify the expected async with lifecycle for a bare connection and add focused coverage for the reported example; done means the example enters and exits without AttributeError.

Written by the indexing model from the issue text.