Someone thought a transient attr would be a clever way to smuggle data
from a _SubParsersAction up to the main ArgumentsParser. Unfortunately
they use a really weird way to set the attr:
vars(namespace).setdefault(_UNRECOGNIZED_ARGS_ATTR, [])
This bypasses __setattr__ so when they try to delattr later,
__delattr__ can't find it and bombs. Even though the delattr is
protected by a hasattr, because hasattr uses getattr which checks the
private object dict first before calling __getattr__.
Handle this by falling back to super in __getattr__ and __delattr__.
Change-Id: Id89a0c00fd32b6580d891ccf69d58fa17f11ec49
Closes-Bug: #1540959
068784afab