Dev docs: support for Debian Bullseye (current stable)

[conorsch]

Describe the change

We maintain developer docs for Debian Stable, which recently (2021-08) transitioned from Debian 10 Buster to Debian 11 Bullseye. Let's run through the dev docs on a Bullseye machine and make any updates as required.

How will this impact users?

Dev-only: the docs should point to the most recent stable version.

Additional context

The libvirt tooling in particular has seen a lot of churn, so make sure to inspect the VM dependencies carefully.

[conorsch]

Performed an in-place upgrade from Buster to Bullseye, and took the dev env for a spin. In short, it worked, although I had to bump the dependency for pyenchant in the dev reqs, due to a problem like that reported in freedomofpress/securedrop-docs#218 Other sundry notes:

• had to destroy and recreate virtualenv, of course, because system python had changed
• had to apt-get remove and apt-get install docker deps based on https://docs.docker.com/engine/install/debian/

That's about it. Was able to build debs in containers, provision staging VMs, run Tails VM, and provision prod VMs via Tails VM.

A better test for the dev docs is a clean install, which will take more time. Will only do so if folks find that useful.

[conorsch]

Earlier today, @tesitura mentioned she encountered pyenchant problems when setting up the dev env on Debian 11. I'll push a branch with a small change and ask her to test, in order to unblock her.