gnu: python2-ipython: Fix inputs.

This fixes e62600feb6, where the inputs
have been changed to native-inputs, but still extended from inputs.

* gnu/packages/python.scm (python2-ipython)[native-inputs]: Extend
from python-ipython's package-native-inputs.

gnu/packages/python.scm

@@ -4814,7 +4814,7 @@ computing.")
       ;; FIXME: add pyreadline once available.
       (native-inputs
        `(("python2-mock" ,python2-mock)
-         ,@(package-inputs ipython))))))
+         ,@(package-native-inputs ipython))))))
 
 (define-public python-isodate
   (package