clean python 3 specific stuff from stdlib/@python2 stubs (#5451)

stdlib/@python2/telnetlib.pyi

@@ -110,6 +110,3 @@ class Telnet:
     def expect(
         self, list: Sequence[Union[Pattern[bytes], bytes]], timeout: Optional[float] = ...
     ) -> Tuple[int, Optional[Match[bytes]], bytes]: ...
-    if sys.version_info >= (3, 6):
-        def __enter__(self) -> Telnet: ...
-        def __exit__(self, type: Any, value: Any, traceback: Any) -> None: ...