plpython: Reject Python 2 during build configuration.

plpython: Reject Python 2 during build configuration.

Python 2.7 went EOL 2020-01-01 and the support for Python 2 requires a fair
bit of infrastructure. Therefore we are removing Python 2 support in plpython.

This patch just rejects Python 2 during configure / mkvcbuild.pl. Future
commits will remove the code and infrastructure for Python 2 support and
adjust more of the documentation. This way we can see the buildfarm state
after the removal sooner and we can be sure that failures are due to
desupporting Python 2, rather than caused by infrastructure cleanup.

Reviewed-By: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/20211031184548.g4sxfe47n2kyi55r@alap3.anarazel.de