Update package build logic to support securedrop-proxy builds on Buster

[eloquence]

Part of epic: freedomofpress/securedrop-workstation#306

#89 adds support for securedrop-client builds on Debian Buster. We need to do the same for securedrop-proxy.

[redshiftzero]

first we need to do a securedrop-proxy release - @creviera want to do that today and i can review?

For background for the interested observer, the reason why we need a securedrop-proxy release before adding buster builds is that the build-requirements.txt in that repository needs to point to the wheels on both 3.5 (stretch) and 3.7 (buster). Fortunately @kushaldas made that change already in freedomofpress/securedrop-proxy#49 so we just need to make a new release including his change ✨

[redshiftzero]

done in #97

(made a card on the board for the package release)