Build broken for ci-base

Trying to build new images today, I get this:

Get:1 http://deb.debian.org/debian bullseye InRelease [116 kB]
Get:2 http://deb.debian.org/debian bullseye/main amd64 Packages [7678 kB]
Fetched 7795 kB in 15s (535 kB/s)
Reading package lists...
Reading package lists...
Building dependency tree...
Reading state information...
Package pyflakes is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  python3-pyflakes pyflakes3

Package python-wheel is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package python-virtualenv is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'pyflakes' has no installation candidate
E: Unable to locate package python-pip
E: Unable to locate package python-subversion
E: Package 'python-virtualenv' has no installation candidate
E: Package 'python-wheel' has no installation candidate
The command '/bin/sh -c apt-get update &&     apt-get install -y --no-install-recommends         build-essential         check-manifest         clang-format         curl         flake8         git         make         mercurial         netbase         pyflakes         pyflakes3         pylint         python         python3         python3-dev         python3-pip         python3-pygments         python3-pytest-cov         python3-setuptools         python3-virtualenv         python3-wheel         python-dev         python-pip         python-pygments         python-setuptools         python-subversion         python-virtualenv         python-wheel         shellcheck         subversion         subversion-tools         unzip         virtualenv     &&     apt-get -y clean' returned a non-zero code: 100
make: *** [Makefile:57: ci-base] Error 100

I didn't look further, but that really looks as a consequence of the removal of Python2 packages in Debian bullseye. Perhaps we can cheat a bit and replace some of them can be replaced by their python3 counterparts or undergo some manual installations.

In the longer run (as long as we'd need to support any Python 2 code), we'd probably have to resplit the base image into py2 and py3 variants, and have the py2 one be based on Debian 10 (buster) instead.