plpythonu -> python3

The docs currently say

The language named <literal>plpythonu</literal> implements
PL/Python based on the default Python language variant, which is
currently Python 2. (This default is independent of what any
local Python installations might consider to be
their <quote>default</quote>, for example,
what <filename>/usr/bin/python</filename> might be.) The
default will probably be changed to Python 3 in a distant future
release of PostgreSQL, depending on the progress of the
migration to Python 3 in the Python community.

As python2 is EOL very soon, I'd say that point is now, i.e. we should
make plpythonu.control point at plpython3u in PG13+. And probably drop
python2 support altogether.

For PG12, I have the problem that I don't want to keep supporting
python2 (Debian is already working hard on removing all python2
references), and have therefore already disabled building the
plpython2 packages for Debian, shipping only plpython3.

PostGIS developer Raúl Marín has rightfully noticed that this leaves
us without the "plpythonu" extension, forcing everyone to move to
"plpython3u" even when their code works with both.

How do other packagers handle that? Are you still supporting python2?
Would it be ok to make plpythonu.control point at python3 in PG12 in
Debian, even the upstream default is still python2?

Christoph