Bug#996584: (some kind of) transition: add python3.10 as a supported python3 version

[Matthias Klose <doko@debian.org>]

I'm planning to upload python3-defaults later tonight, adding 3.10 as a
supported Python version.  Packages are able to migrate on their own, there are
no blockages introduced on other transitions.

We have most packages ready to build for 3.10, and around 70 leaf packages still
needing some work. Otoh, we can much better work on these if reverse
dependencies are already built for 3.10 in the archive.  The tracker used is

https://release.debian.org/transitions/html/python3.10-add.html

As some kind of reference, the current state of the 3.10 addition in Ubuntu can
be seen at
https://people.canonical.com/~ubuntu-archive/transitions/html/python3.10-add.html

Matthias