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.

See this issue on 3.9.6
No SSL, but plain sockets.
This seems to appear when writer.write/writer.drain was cancelled, and writer.close/writer.wait_closed called after this.

On main branch with rewritten SSL implementation GH-31275 bpo-44011, this raises TimeoutError:

--------------------------------------------------------------------------------
opened
close started

Traceback (most recent call last):
  File "/workspaces/cpython/main.py", line 18, in <module>
    asyncio.run(main())
    ^^^^^^^^^^^^^^^^^^^
  File "/workspaces/cpython/Lib/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspaces/cpython/Lib/asyncio/base_events.py", line 645, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/workspaces/cpython/main.py", line 14, in main
    await writer.wait_closed()
    ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspaces/cpython/Lib/asyncio/streams.py", line 344, in wait_closed
    await self._protocol._get_close_waiter(self)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TimeoutError: SSL shutdown timed out
--------------------------------------------------------------------------------

This issue has been fixed, it can be closed @asvetlov.