Use Self TypeVar for SSHConnection __aenter__ (#600)

* Use Self TypeVar for SSHConnection __aenter__

This makes it so that when using a subclass as a context manager like so:

async with asyncssh.connect(...) as con:

the con variable has the type SSHClientConnection instead of simply SSHConnection.

asyncssh/connection.py

@@ -179,6 +179,7 @@
 _ProtocolFactory = Union[_ClientFactory, _ServerFactory]
 
 _Conn = TypeVar('_Conn', 'SSHClientConnection', 'SSHServerConnection')
+_ConnSelf = TypeVar('_ConnSelf', bound='SSHConnection')
 
 class _TunnelProtocol(Protocol):
     """Base protocol for connections to tunnel SSH over"""
@@ -944,7 +945,7 @@ def __init__(self, loop: asyncio.AbstractEventLoop,
 
         self._disable_trivial_auth = False
 
-    async def __aenter__(self) -> 'SSHConnection':
+    async def __aenter__(self: _ConnSelf) -> _ConnSelf:
         """Allow SSHConnection to be used as an async context manager"""
 
         return self