Raising an issue that's impacting us on `httpx`.

It appears that in some cases SSL unwrapping can cause `.wait_closed()` to hang indefinately.

Trio are particularly careful to work around this case, and have an extensive comment on it: https://github.com/python-trio/trio/blob/31e2ae866ad549f1927d45ce073d4f0ea9f12419/trio/_ssl.py#L779-L829 

Originally raised via https://github.com/encode/httpx/issues/634

Tested on:

* Python 3.7.6
* Python 3.8.1

```
import asyncio
import ssl
import certifi

hostname = 'login.microsoftonline.com'
context = ssl.create_default_context()
context.load_verify_locations(cafile=certifi.where())

async def main():
    reader, writer = await asyncio.open_connection(hostname, 443, ssl=context)
    print('opened')
    writer.close()
    print('close started')
    await writer.wait_closed()
    print('close completed')

asyncio.run(main())
```

Can reproduce in:

- 3.8.0
- 3.8.2
- 3.7.6

I am also seeing this in Python 3.8.6. I am not using SSL, but am simply calling `await writer.wait_closed()` on an `asyncio.StreamWriter`. Sometimes it works quickly and sometimes it hangs indefinitely.

Regarding my previous comment: I have never seen this in Python 3.7 (though I see that this particular bug is listed as being present there) so it may be a different underlying issue.