Updates CI to run on Buster for Debian packaging

[kushaldas]

Description

Fixes #527

Before this freedomofpress/securedrop-builder#89 needs to get merged. Then we will have to kick the CI for rebuild.

Test Plan

• Get the branch from Updates to Python 3.7 on Buster for package building for securedrop-client securedrop-builder#89 and then try to build the package.

./update_version.sh 1000.0  # Dummy version number, doesn't matter what we put here python3 setup.py sdist
cd ../securedrop-debian-packaging
export PKG_VERSION=1000.0
export PKG_PATH=/home/circleci/project/dist/securedrop-client-$PKG_VERSION.tar.gz
make securedrop-client

[eloquence]

The build-stretch failure reported in CI is real as the job no longer exists in the config file.

My understanding was that we were going to keep the strech build job alongside the buster build job, same as we did for test-stretch & test-buster. Is that incorrect?

[kushaldas]

After we revert freedomofpress/securedrop-builder#89 and redo the PR there, we can merge this one here.

[kushaldas]

Kicked the CI to rerun.

[redshiftzero]

want to make these run steps reusable by adding them to common-steps above as YAML anchors and then referencing them here and in the below build-buster job? (that way we keep these CI configs DRY)

[kushaldas]

I will make this change, I never knew about YAML anchors before.

[redshiftzero]

did you get a chance to look at this @kushaldas ?

[kushaldas]

did you get a chance to look at this @kushaldas ?

Nope, will finish that in the morning.

[kushaldas]

did you get a chance to look at this @kushaldas ?

Updated the PR with the suggestions.

[redshiftzero]

lgtm!