... so move it to python/ExternalProject_python3.mk, where it belongs.

Change-Id: Ib99a6a40182341257f79dd289eac51806be46fcf
Reviewed-on: https://gerrit.libreoffice.org/3778Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>