If an asyncio SSL connection is left open (eg. any kind of keep-alive connection) then after closing the event loop, an exception will be raised...

Python:

```
import asyncio
import ssl
import certifi


async def f():
    ssl_context = ssl.create_default_context()
    ssl_context.load_verify_locations(cafile=certifi.where())
    await asyncio.open_connection('example.org', 443, ssl=ssl_context)


loop = asyncio.get_event_loop()
loop.run_until_complete(f())
loop.close()
```

Traceback:

```
$ python example.py 
Fatal write error on socket transport
protocol: <asyncio.sslproto.SSLProtocol object at 0x10e7874a8>
transport: <_SelectorSocketTransport fd=8>
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/selector_events.py", line 868, in write
    n = self._sock.send(data)
OSError: [Errno 9] Bad file descriptor
Fatal error on SSL transport
protocol: <asyncio.sslproto.SSLProtocol object at 0x10e7874a8>
transport: <_SelectorSocketTransport closing fd=8>
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/selector_events.py", line 868, in write
    n = self._sock.send(data)
OSError: [Errno 9] Bad file descriptor

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/sslproto.py", line 676, in _process_write_backlog
    self._transport.write(chunk)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/selector_events.py", line 872, in write
    self._fatal_error(exc, 'Fatal write error on socket transport')
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/selector_events.py", line 681, in _fatal_error
    self._force_close(exc)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/selector_events.py", line 693, in _force_close
    self._loop.call_soon(self._call_connection_lost, exc)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/base_events.py", line 677, in call_soon
    self._check_closed()
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/base_events.py", line 469, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed
```

It looks to me like the original "OSError: [Errno 9] Bad file descriptor" probably shouldn't be raised in any case - if when attempting to tear down the SSL connection, then we should probably pass silently in the case that the socket has already been closed uncleanly.

Bought to my attention via: https://github.com/encode/httpcore/issues/16