Previously there was no way to reliably run the default setattr
behavior, which can be requested by calling object.__setattr__
or PyObject_GenericSetattr.  When those were called, we would
check to see if there is a custom __setattr__ defined and then
try to call that.  But users can and do define custom __setattr__
functions that defer to object.__setattr__, so we need a way to
run the default behavior without deferring to a custom setattr.