Buildfarm update to Python 3 needed

Per recent discussions on pgsql-hackers [1], we are intending
to desupport Python 2 in Postgres beginning in v15. This means
that for buildfarm animals to build/test plpython, it will soon
be necessary for them to have a Python 3 installation.

If there is already a "python3" available in the animal's PATH,
you don't need to do anything; it will automatically start using
that when the switchover is made. Otherwise, you can either
install Python 3.x, or remove "--with-python" from the animal's
configuration.

If you're manually forcing choice of a particular Python
installation by setting the PYTHON environment variable, you'll
need to adjust that variable to point at a Python 3 installation.

Note that there's no need to change which Python is used for
building v14 and earlier, though you're free to do so if you
prefer.

Which 3.x minor version to use is mostly up to you. Be aware
however that there's been discussion of switching our build
system from autoconf/gmake to meson/ninja; if that happens,
it'll create a requirement to have at least Python 3.6.

We anticipate making this change in mid-February, so you have
a month to do whatever needs to be done with your buildfarm
animals.

regards, tom lane

[1] https://www.postgresql.org/message-id/flat/20211031184548.g4sxfe47n2kyi55r%40alap3.anarazel.de