Add undocumented telnetlib.Telnet attribute. (#4932)

A pytype user reported it missing:
https://github.com/google/pytype/issues/590.

stdlib/2and3/telnetlib.pyi

@@ -82,6 +82,7 @@ EXOPL: bytes
 NOOPT: bytes
 
 class Telnet:
+    host: Optional[str]  # undocumented
     def __init__(self, host: Optional[str] = ..., port: int = ..., timeout: int = ...) -> None: ...
     def open(self, host: str, port: int = ..., timeout: int = ...) -> None: ...
     def msg(self, msg: str, *args: Any) -> None: ...