[CI] Update container images for tests

[conorsch]

We're currently using the circleci/python-3.7 images in CI. According to the CircleCI docs:

Legacy images with the prefix "circleci/" will be deprecated on December 31, 2021. For faster builds, upgrade your projects with next-generation convenience images.

The recommended image would be cimg/python, but that's based on Ubuntu Focal 20.04, whereas we need Debian 10 (until we migrate to Debian 11 for the Workstation). Let's consider debian:buster instead.

[conorsch]

@gonzalo-bulnes started a branch to resolve this, which I've picked up and appended to. Tracking in https://github.com/freedomofpress/securedrop-client/pull/1385, with the optimistic goal of having this week, ahead of the "deprecation" date at EOY.

[conorsch]

Recording runtimes for historical CI jobs, for reference:

• test-buster: 5-7m
• build-buster: 3-5m

It's possible, even likely, that switching the container image will affect wall time on CI, so let's evaluate that as part of the change.

[conorsch]

Based on discussion in #1385, we'll use debian:buster as the default, and explicitly add packages required for a build environment. I've updated the OP for s/python:3.7-buster/debian:buster/ to match.

[legoktm]

We've since standardized on plain debian:{version} images.