Python 2 support for Bullseye

[Moritz Mühlenhoff <jmm@inutil.org>]

There will be few core packages build-depending on Python 2 (for tests
or building) which won't be ready for Python 3 for Bullseye (Chromium,
qtwebkit and IIRC also Pypy), but those only need Python 2 (and a very
small set of support packages like setuptools/jinja) to build and
run their tests.

Apart from those there's only a handful of packages still in testing
which have a run time dependency on Python 2 packages (and most are
actively being worked on (e.g. Xen)).

All packages (and their test suites) shipped in Debian are trusted
anyway (and consequently don't need any kind of updates during the
bullseye cycle), so a lack of updates/upstream EOL doesn't matter.

As such, I'd propose to include Python 2 (plus the small set of
support packages) in Bullseye but exempt it from support (and then
remove it for good after the Bullseye release):
- Mark src:python (plus related support packages) as unsupported in
  debian-security-support and with a README.Debian in the source
  package (and given how prominent Python is, also in the release notes)
- Bump the severity of the few remaining packages in testing to RC
  state (and force them out testing if auto removals were blocked
  for some reason)

Thoughts?

Cheers,
        Moritz