autobuild: We should run autobuild with python3 by default

autobuild: We should run autobuild with python3 by default

sn-devel autobuild runing autobuild.py (via git hooks I suppose) but
if run directly (e.g. depending on script shebang) then 'python' aka
python2 will run. This will cause an error when building some targets
because the autobuild script itself sometimes builds paths based
on the version of python executing the script e.g ${PYTHON_PREFIX}.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>