fdo#70393: move python3 to a subdir of external

Change-Id: Ic5796f096255d2d84e39415324e8a2e06bcf09c9
Reviewed-on: https://gerrit.libreoffice.org/6550
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>