Python 3.9 for Ubuntu 21.04 (h-series)

[Matthias Klose]

Python 3.9 will be the default Python3 version for 21.04 (h-series).  A year ago
we prepared for 3.8 as the default in focal, going ahead of Debian.  For the
transition to 3.9, we started PPA test rebuilds [1] [2] while groovy was frozen,
and submitted bug reports for Debian [3], before starting the 3.9 addition in
Debian [4].  With the opening of the h-series, we will add 3.9 as a supported
Python3 version [5], and then making it the default, hopefully before the end of
the year.

Matthias

[1] https://launchpad.net/~pythoneers/+archive/ubuntu/python3.9/+packages
[2] https://launchpad.net/~pythoneers/+archive/ubuntu/python3.9-fixes/+packages
[3]
https://bugs.debian.org/cgi-bin/pkgreport.cgi?tag=python3.9;users=debian-python@lists.debian.org
[4] https://release.debian.org/transitions/html/python3.9.html
[5] https://people.canonical.com/~ubuntu-archive/transitions/html/python3.9-add.html