Use python3 for system tests
Use python3 for system tests

Most modern distribution use python3 for python (3). Default to that to
make it a bit simpler to run systems tests on Debian

Signed-off-by: Sjoerd Simons <1dafb4c4a7acdc6e1eca32ff35fda03e0e9cd976@collabora.com>

aptly-dev/aptly,aptly-dev/aptly,aptly-dev/aptly