obsoleting plpython2u and defaulting plpythonu to plpython3u

Per current plpython docs:

The language named plpythonu 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 “default”, for example, what /usr/bin/python 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.

.. the status quo seems to be bit optimistic with the "distant future",
and we should start thinking about dropping plpython2 support, same as
upstream (a bit optimistically too, IMO) does [1].

The docs don't suggest the explicit use of plpython2, but still the docs
are not discouraging from it -- so it is likely some clusters run against
that.

What's the expected future migration path from plpython2 to plpython3 in
such cases? I'm thinking about rewrite of the docs and creating some
scripting which could simplify the migration steps. Would such patches be
welcome at this point?

[1] https://pythonclock.org/

Pavel