Add release for debian/buster

[lingxiaoyang]

Currently, packagecloud is only offering debian/stretch and this leads to an installation failure following the installation instructions on debian/buster.

debian/buster has been the backing environment for a number of standard Docker images, such as python3.7.

[nickatsegment]

FYI the DEBs are identical, so you can just download and install with dpkg -i. Of course you don't get automatic updates with that.

wontfix due to #278