> 
> python.el is on MELPA? (and it /does/ use Compat!)  But note that Compat
> doesn't implement everything, IIRC this includes
> derived-mode-add-parents.


Sorry, I was confusing the python-mode on MELPA.  The issue is actually with the version on gnu-devel: 

https://elpa.gnu.org/devel/python.html