Build & test system on Bullseye and newer: use custom, fake, unversioned python packages

The unversioned python packages are not shipped in Bullseye/sid anymore,
and even old versions are not installable anymore. This makes
vmdebootstrap and some dependencies of our test suite
(#15349, #17031) uninstallable.

So, I've uploaded to our custom APT repo some fake unversioned python packages,
which allow installing a Tails build & test environment on Bullseye/sid.

The control files added by this commit are used to generate these fake packages
with equivs-build.

While I'm at it, I'm documenting how to install vmdebootstrap on Bullseye
and newer.

refs #17858

config/equivs/.gitignore

+*.deb
+*.buildinfo
+*.changes

config/equivs/libpython-stdlib.control

+Section: python
+Priority: optional
+Homepage: https://tails.boum.org/
+Standards-Version: 3.9.6
+
+Package: libpython-stdlib
+Version: 2.7.17-2.0tails1
+Maintainer: Tails developers <tails@boum.org>
+Architecture: amd64
+Multi-Arch: allowed
+Depends: libpython2-stdlib
+Description: (Fake) libpython-stdlib
+ Dummy package that meet the dependencies of the obsolete bits of Tails'
+ build & test system (#15349, #17031).

config/equivs/python-minimal.control

+Section: python
+Priority: standard
+Homepage: https://tails.boum.org/
+Standards-Version: 3.9.6
+
+Package: python-minimal
+Version: 2.7.17-2.0tails1
+Maintainer: Tails developers <tails@boum.org>
+Architecture: amd64
+Multi-Arch: allowed
+Depends: python2-minimal
+Description: (Fake) python-minimal
+ Dummy package that meet the dependencies of the obsolete bits of Tails'
+ build & test system (#15349, #17031).

config/equivs/python.control

+Section: python
+Priority: standard
+Homepage: https://tails.boum.org/
+Standards-Version: 3.9.6
+
+Package: python
+Version: 2.7.17-2.0tails1
+Maintainer: Tails developers <tails@boum.org>
+Architecture: amd64
+Multi-Arch: allowed
+Depends: python2
+Description: (Fake) python
+ Dummy package that meet the dependencies of the obsolete bits of Tails'
+ build & test system (#15349, #17031).

wiki/src/contribute/build.mdwn

@@ -22,6 +22,13 @@ To build Tails you need:
    - 5 GB in `/var/lib` if the system has at least 16 GiB of RAM;
      else, 20 GB in `/var/lib`
 
+Then:
+
+* If you are using Debian Bullseye or newer:
+
+   - Add an extra APT source: `deb http://deb.tails.boum.org/ builder-bullseye main`
+   - Enable the Buster APT sources, to install `vmdebootstrap` and its dependencies.
+
 # Setup the build environment
 
 1. To install everything the Tails build system needs, execute the

wiki/src/contribute/release_process/test/setup.mdwn

@@ -50,6 +50,9 @@ To install the dependencies on our test suite:
             # For python-jabberbot and python-potr, that were removed after Buster
             echo 'deb http://ftp.us.debian.org/debian/ buster main' \
                 | sudo tee /etc/apt/sources.list.d/buster.list
+            # For unversioned python packages, needed by the above
+            echo 'deb http://deb.tails.boum.org/ isotester-bullseye main' \
+                | sudo tee /etc/apt/sources.list.d/isotester-bullseye.list
         fi
         if [ "${dist}" = buster ]; then
             # For our qemu 4.2 buster-backport