Fix optparse OptionGroup __init__ to allow missing description (#1248)

Author: jdb8

stdlib/2and3/optparse.pyi

@@ -152,7 +152,7 @@ class OptionGroup(OptionContainer):
     option_list = ...  # type: List[Option]
     parser = ...  # type: OptionParser
     title = ...  # type: _Text
-    def __init__(self, parser: OptionParser, title: _Text, description: Optional[_Text]) -> None: ...
+    def __init__(self, parser: OptionParser, title: _Text, description: Optional[_Text] = ...) -> None: ...
     def _create_option_list(self) -> None: ...
     def set_title(self, title: _Text) -> None: ...