Add __init__ and __contains__ to argparse.Namespace (#1578)

Author: rmcgibbo

stdlib/2and3/argparse.pyi

@@ -117,8 +117,10 @@ class Action:
                  option_string: _Text = ...) -> None: ...
 
 class Namespace:
+    def __init__(self, **kwargs: Any) -> None: ...
     def __getattr__(self, name: _Text) -> Any: ...
     def __setattr__(self, name: _Text, value: Any) -> None: ...
+    def __contains__(self, key: str) -> bool: ...
 
 class FileType:
     if sys.version_info >= (3, 4):