Downloading: http://repo.aptly.info/system-tests/archive.debian.org/debian-archive/debian/dists/stretch-backports/InRelease
openpgp:  RSA key ID 648ACFD622F3D138
openpgp: Good signature from "Debian Archive Automatic Signing Key (10/buster) <ftpmaster@debian.org>"
openpgp:  RSA key ID 0E98404D386FA1D9
openpgp: Good signature from "Debian Archive Automatic Signing Key (11/bullseye) <ftpmaster@debian.org>"

Mirror [mirror9]: http://repo.aptly.info/system-tests/archive.debian.org/debian-archive/debian/ stretch-backports successfully added.
You can run 'aptly mirror update mirror9' to download repository contents.