retitle [PATCH core-updates-frozen] build-system-python: Change let* to let.
thanks

>[python-build-system.scm]
> -  (let* ((out (python-output outputs))
> -         (python (assoc-ref inputs "python")))
> +  (let ((out (python-output outputs))
> +        (python (assoc-ref inputs "python")))

According to "guix refresh -l python-six", this would rebuild a lot
(at least 3122), so this would need to be done on core-updates.  Seems
ok otherwise.