Downloading http://mirror.yandex.ru/debian/dists/stretch-backports/InRelease...
gpgv:  RSA key ID 2B90D010
gpgv: Good signature from "Debian Archive Automatic Signing Key (8/jessie) <ftpmaster@debian.org>"
gpgv:  RSA key ID B7D453EC
gpgv: Good signature from "Debian Archive Automatic Signing Key (9/stretch) <ftpmaster@debian.org>"

Mirror [mirror9]: http://mirror.yandex.ru/debian/ stretch-backports successfully added.
You can run 'aptly mirror update mirror9' to download repository contents.