Add default for SFTPClient.listdir

At runtime, it's possible to call `listdir()` (i.e. without arguments)
and get a `Sequence[str]` back, due to the default argument in the
implementation. However, there is no `@overload` that contains 0
arguments, so mypy doesn't think that it's possible.

This adds the presence of the default to the sequence-of-strings
returning overload, to match the implementation

asyncssh/sftp.py

@@ -5168,7 +5168,7 @@ async def listdir(self, path: bytes) -> \
         Sequence[bytes]: ... # pragma: no cover
 
     @overload
-    async def listdir(self, path: FilePath) -> \
+    async def listdir(self, path: FilePath = ...) -> \
         Sequence[str]: ... # pragma: no cover
 
     async def listdir(self, path: _SFTPPath = '.') -> Sequence[BytesOrStr]: