lib.FileContext.open(): Add method

Add an async open() method which should allow to do what __init__()
couldn't, because it's not async, and to match the already existing
.close(). It's called by __aenter__() __aexit__() if the FileContext
is instantiated as context manager, or at will when the user finds it
a good idea.

Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/lib/ec/ssh/AsyncSSH.py

@@ -35,6 +35,10 @@ class AsyncSSH(Base):
         self.__connect_timeout = connect_timeout
         self.__conn: asyncssh.SSHClientConnection|None = None
 
+    async def _open(self) -> None:
+        await super()._open()
+        await self._conn
+
     async def _close(self) -> None:
         if self.__conn is not None:
             try: